I am attempting to cleanup 'prepend mbuf allocations' and have included 
a diff below.

These sendto's are the most frequest now that wireguard fixes are in.
I assume these are mostly unbound doing DNS.

m_prepend: len 48
-+- mbuf 0xfffffd8013662000, dat, off 0, len 33, pktlen 33, size 144
  \\- total chain 1, len 33, size 144
Starting stack trace...
m_print_debug(fffffd8013662000,ffffffff819eb167,30) at m_print_debug+0x31d
m_prepend(fffffd8013662000,30,2) at m_prepend+0xbe
udp6_output(fffffd824dab66d0,fffffd8013662000,0,0) at udp6_output+0x167
sosend(ffff8000003b2040,0,ffff80003172d0c0,0,0,80) at sosend+0x3e2
sendit(ffff80003162e540,6,ffff80003172d1b8,0,ffff80003172d250) at 
sendit+0x395
sys_sendto(ffff80003162e540,ffff80003172d2e0,ffff80003172d250) at 
sys_sendto+0x6c
syscall(ffff80003172d2e0) at syscall+0x620
Xsyscall() at Xsyscall+0x128
end of kernel
end trace frame: 0x7cc803157ff0, count: 249
End of stack trace.

m_prepend: len 28
-+- mbuf 0xfffffd800a2f5a00, dat, off 0, len 23, pktlen 23, size 144
   \- total chain 1, len 23, size 144
Starting stack trace...
m_print_debug(fffffd800a2f5a00,ffffffff819e7e0d,1c) at m_print_debug+0x31d
m_prepend(fffffd800a2f5a00,1c,2) at m_prepend+0xbe
udp_output(fffffd820aef7938,fffffd800a2f5a00,fffffd801483b600,0) at
udp_output+0x2f2
sosend(ffff80000089e548,fffffd801483b600,ffff800031614ff0,0,0,80) at
sosend+0x3e2
sendit(ffff80003161c2a8,3,ffff8000316150e8,0,ffff800031615180) at
sendit+0x395
sys_sendto(ffff80003161c2a8,ffff800031615210,ffff800031615180) at
sys_sendto+0x6c
syscall(ffff800031615210) at syscall+0x620
Xsyscall() at Xsyscall+0x128
end of kernel
end trace frame: 0x7560c50f07b0, count: 249
End of stack trace.

The following diff does not produce any of the debugs above.  Please 
tell me if this is correct or have caused another issue.

1.) on first iteration, only write at most (MHLEN-max_hdr) bytes, saving
room for max_hdr.

2.) since MINCLSIZE > (MHLEN-max_hdr), delete the code to move m_data 
forward since it m will never equal top here.

3.) fix off by one in comparison.

Index: kern/uipc_socket.c
===================================================================
RCS file: /cvs/src/sys/kern/uipc_socket.c,v
diff -u -p -r1.344 uipc_socket.c
--- kern/uipc_socket.c	31 Oct 2024 12:51:55 -0000	1.344
+++ kern/uipc_socket.c	31 Oct 2024 17:41:27 -0000
@@ -795,6 +795,7 @@ m_getuio(struct mbuf **mp, int atomic, l
  		if (top == NULL) {
  			MGETHDR(m, M_WAIT, MT_DATA);
  			mlen = MHLEN;
+			resid = ulmin(resid, MHLEN - max_hdr);
  		} else {
  			MGET(m, M_WAIT, MT_DATA);
  			mlen = MLEN;
@@ -812,12 +813,6 @@ m_getuio(struct mbuf **mp, int atomic, l
  				goto nopages;
  			mlen = m->m_ext.ext_size;
  			len = ulmin(mlen, resid);
-			/*
-			 * For datagram protocols, leave room
-			 * for protocol headers in first mbuf.
-			 */
-			if (atomic && m == top && len < mlen - max_hdr)
-				m->m_data += max_hdr;
  		} else {
  nopages:
  			len = ulmin(mlen, resid);
@@ -825,7 +820,7 @@ nopages:
  			 * For datagram protocols, leave room
  			 * for protocol headers in first mbuf.
  			 */
-			if (atomic && m == top && len < mlen - max_hdr)
+			if (atomic && m == top && len <= mlen - max_hdr)
  				m_align(m, len);
  		}