Backport commit from NetBSD with commit message:

Avoid Undefined Behavior in ffs_clusteracct()

Change the type of 'bit' variable from int to unsigned int and use unsigned
values consistently.

sys/ufs/ffs/ffs_subr.c:336:10, shift exponent -1 is negative

Detected with Kernel Undefined Behavior Sanitizer.

Tested on amd64 over days. OK?

diff --git a/sys/ufs/ffs/ffs_alloc.c b/sys/ufs/ffs/ffs_alloc.c
index 14822193a03..9363637deee 100644
--- a/sys/ufs/ffs/ffs_alloc.c
+++ b/sys/ufs/ffs/ffs_alloc.c
@@ -1478,7 +1478,8 @@ ffs_clusteracct(struct fs *fs, struct cg *cgp, daddr_t blkno, int cnt)
 	int32_t *sump;
 	int32_t *lp;
 	u_char *freemapp, *mapp;
-	int i, start, end, forw, back, map, bit;
+	int i, start, end, forw, back, map;
+	u_int bit;
 
 	if (fs->fs_contigsumsize <= 0)
 		return;
@@ -1500,7 +1501,7 @@ ffs_clusteracct(struct fs *fs, struct cg *cgp, daddr_t blkno, int cnt)
 		end = cgp->cg_nclusterblks;
 	mapp = &freemapp[start / NBBY];
 	map = *mapp++;
-	bit = 1 << (start % NBBY);
+	bit = 1U << (start % NBBY);
 	for (i = start; i < end; i++) {
 		if ((map & bit) == 0)
 			break;
@@ -1521,7 +1522,7 @@ ffs_clusteracct(struct fs *fs, struct cg *cgp, daddr_t blkno, int cnt)
 		end = -1;
 	mapp = &freemapp[start / NBBY];
 	map = *mapp--;
-	bit = 1 << (start % NBBY);
+	bit = 1U << (start % NBBY);
 	for (i = start; i > end; i--) {
 		if ((map & bit) == 0)
 			break;
@@ -1529,7 +1530,7 @@ ffs_clusteracct(struct fs *fs, struct cg *cgp, daddr_t blkno, int cnt)
 			bit >>= 1;
 		} else {
 			map = *mapp--;
-			bit = 1 << (NBBY - 1);
+			bit = 1U << (NBBY - 1);
 		}
 	}
 	back = start - i;