BRT: Skip getting length in brt_entry_lookup()

Unlike DDT, where ZAP values may have different lengths due to
compression, all BRT entries are identical 8-byte counters.  It
does not make sense to first fetch the length only to assert it.
zap_lookup_uint64() is specifically designed to work with counters
of different size and should return error if something odd found.
Calling it straight allows to save some measurable CPU time.

Reviewed-by: Pawel Jakub Dawidek <pawel@dawidek.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#15950

module/zfs/brt.c

@@ -900,7 +900,6 @@ static int
 brt_entry_lookup(brt_t *brt, brt_vdev_t *brtvd, brt_entry_t *bre)
 {
 	uint64_t mos_entries;
-	uint64_t one, physsize;
 	int error;
 
 	ASSERT(RW_LOCK_HELD(&brt->brt_lock));
@@ -918,21 +917,8 @@ brt_entry_lookup(brt_t *brt, brt_vdev_t *brtvd, brt_entry_t *bre)
 
 	brt_unlock(brt);
 
-	error = zap_length_uint64(brt->brt_mos, mos_entries, &bre->bre_offset,
-	    BRT_KEY_WORDS, &one, &physsize);
-	if (error == 0) {
-		ASSERT3U(one, ==, 1);
-		ASSERT3U(physsize, ==, sizeof (bre->bre_refcount));
-
-		error = zap_lookup_uint64(brt->brt_mos, mos_entries,
-		    &bre->bre_offset, BRT_KEY_WORDS, 1,
-		    sizeof (bre->bre_refcount), &bre->bre_refcount);
-		BRT_DEBUG("ZAP lookup: object=%llu vdev=%llu offset=%llu "
-		    "count=%llu error=%d", (u_longlong_t)mos_entries,
-		    (u_longlong_t)brtvd->bv_vdevid,
-		    (u_longlong_t)bre->bre_offset,
-		    error == 0 ? (u_longlong_t)bre->bre_refcount : 0, error);
-	}
+	error = zap_lookup_uint64(brt->brt_mos, mos_entries, &bre->bre_offset,
+	    BRT_KEY_WORDS, 1, sizeof (bre->bre_refcount), &bre->bre_refcount);
 
 	brt_wlock(brt);