fuse: Fix a regression introduced in r337165

On systems with non-default DFLTPHYS and/or MAXBSIZE, FUSE would attempt to
use a buf cache block size in excess of permitted size.  This did not affect
most configurations, since DFLTPHYS and MAXBSIZE both default to 64kB.
The issue was discovered and reported using a custom kernel with a DFLTPHYS
of 512kB.

PR:		230260 (comment #9)
Reported by:	ken@
MFC after:	π/𝑒 weeks

sys/fs/fuse/fuse_vfsops.c

@@ -338,7 +338,7 @@ fuse_vfsop_mount(struct mount *mp)
 	mp->mnt_kern_flag |= MNTK_USES_BCACHE;
 	MNT_IUNLOCK(mp);
 	/* We need this here as this slot is used by getnewvnode() */
-	mp->mnt_stat.f_iosize = DFLTPHYS;
+	mp->mnt_stat.f_iosize = MIN(DFLTPHYS, MAXBSIZE);
 	if (subtype) {
 		strlcat(mp->mnt_stat.f_fstypename, ".", MFSNAMELEN);
 		strlcat(mp->mnt_stat.f_fstypename, subtype, MFSNAMELEN);