The following diff removes unused pmap_virtual_space() from amd64 and
makes sure either the uvm headers provide a prototype for
pmap_virtual_space() or pmap_steal_memory(), but not both, since the
code path involving pmap_virtual_space() is only compiled when
pmap_steal_memory() does not exist.

With a comment fix as a bonus.

Index: sys/arch/amd64/amd64/pmap.c
===================================================================
RCS file: /cvs/src/sys/arch/amd64/amd64/pmap.c,v
retrieving revision 1.165
diff -u -p -r1.165 pmap.c
--- sys/arch/amd64/amd64/pmap.c	29 Dec 2023 13:23:27 -0000	1.165
+++ sys/arch/amd64/amd64/pmap.c	31 Mar 2024 16:35:19 -0000
@@ -3105,13 +3105,6 @@ pmap_steal_memory(vsize_t size, vaddr_t 
 	return (va);
 }
 
-void
-pmap_virtual_space(vaddr_t *vstartp, vaddr_t *vendp)
-{
-	*vstartp = virtual_avail;
-	*vendp = VM_MAX_KERNEL_ADDRESS;
-}
-
 /*
  * pmap_convert
  *
Index: sys/arch/i386/i386/pmap.c
===================================================================
RCS file: /cvs/src/sys/arch/i386/i386/pmap.c,v
retrieving revision 1.222
diff -u -p -r1.222 pmap.c
--- sys/arch/i386/i386/pmap.c	13 Apr 2023 15:23:22 -0000	1.222
+++ sys/arch/i386/i386/pmap.c	31 Mar 2024 16:35:19 -0000
@@ -1555,7 +1555,7 @@ pmap_extract_86(struct pmap *pmap, vaddr
 }
 
 /*
- * pmap_virtual_space: used during bootup [pmap_steal_memory] to
+ * pmap_virtual_space: used during bootup [uvm_pageboot_alloc] to
  *	determine the bounds of the kernel virtual address space.
  */
 
Index: sys/uvm/uvm_pmap.h
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_pmap.h,v
retrieving revision 1.33
diff -u -p -r1.33 uvm_pmap.h
--- sys/uvm/uvm_pmap.h	13 Apr 2023 15:23:23 -0000	1.33
+++ sys/uvm/uvm_pmap.h	31 Mar 2024 16:35:19 -0000
@@ -173,9 +173,10 @@ void		 pmap_update(pmap_t);
 void		 pmap_zero_page(struct vm_page *);
 #endif
 
-void		 pmap_virtual_space(vaddr_t *, vaddr_t *);
 #if defined(PMAP_STEAL_MEMORY)
 vaddr_t		 pmap_steal_memory(vsize_t, vaddr_t *, vaddr_t *);
+#else
+void		 pmap_virtual_space(vaddr_t *, vaddr_t *);
 #endif
 
 /* nested pmaps are used in i386/amd64 vmm */