vfs_register: prevent kernel crash

vfs_register hashes the filesystem name and uses it for sysctl oids.
A filesystem name which hashes to 0 crashes in sysctl_register_oid().

Map 0 to 1 to prevent the kernel crash.

This can be tested with "udf2" as the filesystem name.

MFC after:	1 month
MFC to:		stable/15 stable/14
Reviewed by:	kib
Differential Revision: https://reviews.freebsd.org/D59839

(cherry picked from commit 3897678132)

sys/kern/vfs_init.c

@@ -427,6 +427,8 @@ vfs_register(struct vfsconf *vfc)
 		 */
 		hashval = fnv_32_str(vfc->vfc_name, FNV1_32_INIT);
 		hashval &= 0xff;
+		if (hashval == 0)
+			hashval = 1;
 		secondpass = 0;
 		do {
 			/* Look for and fix any collision. */