FreeBSD: Fix a pair of bugs in zfs_fhtovp()

- Add a zfs_exit() call in an error path, otherwise a lock is leaked.
- Remove the fid_gen > 1 check.  That appears to be Linux-specific:
  zfsctl_snapdir_fid() sets fid_gen to 0 or 1 depending on whether the
  snapshot directory is mounted.  On FreeBSD it fails, making snapshot
  dirs inaccessible via NFS.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Andriy Gapon <avg@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Fixes: 43dbf88 ("FreeBSD: vfsops: use setgen for error case")
Closes openzfs#14001
Closes openzfs#13974

module/os/freebsd/zfs/zfs_vfsops.c

@@ -1818,7 +1818,8 @@ zfs_fhtovp(vfs_t *vfsp, fid_t *fidp, int flags, vnode_t **vpp)
 		return (SET_ERROR(EINVAL));
 	}
 
-	if (fidp->fid_len == LONG_FID_LEN && (fid_gen > 1 || setgen != 0)) {
+	if (fidp->fid_len == LONG_FID_LEN && setgen != 0) {
+		zfs_exit(zfsvfs, FTAG);
 		dprintf("snapdir fid: fid_gen (%llu) and setgen (%llu)\n",
 		    (u_longlong_t)fid_gen, (u_longlong_t)setgen);
 		return (SET_ERROR(EINVAL));