BRT: Check bv_mos_entries in brt_entry_lookup()

When vdev first sees some block cloning, there is a window when
brt_maybe_exists() might already return true since something was
cloned, but bv_mos_entries is still 0 since BRT ZAP was not yet
created.  In such case we should not try to look into the ZAP
and dereference NULL bv_mos_entries_dnode.

Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.

module/zfs/brt.c

@@ -859,6 +859,9 @@ brt_entry_lookup(brt_vdev_t *brtvd, brt_entry_t *bre)
 {
 	uint64_t off = BRE_OFFSET(bre);
 
+	if (brtvd->bv_mos_entries == 0)
+		return (SET_ERROR(ENOENT));
+
 	return (zap_lookup_uint64_by_dnode(brtvd->bv_mos_entries_dnode,
 	    &off, BRT_KEY_WORDS, 1, sizeof (bre->bre_count), &bre->bre_count));
 }