Use SET_ERROR for constant non-zero return codes

Update many return and assignment statements to follow the convention
of using the SET_ERROR macro when returning a hard-coded non-zero
value from a function. This aids debugging by recording the error
codes in the debug log.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #6441

module/zfs/zap.c

@@ -525,7 +525,7 @@ zap_get_leaf_byblk(zap_t *zap, uint64_t blkid, dmu_tx_t *tx, krw_t lt,
 	 * already be freed, so this should be perfectly fine.
 	 */
 	if (blkid == 0)
-		return (ENOENT);
+		return (SET_ERROR(ENOENT));
 
 	dn = dmu_buf_dnode_enter(zap->zap_dbuf);
 	err = dmu_buf_hold_by_dnode(dn,
@@ -767,7 +767,7 @@ fzap_checksize(uint64_t integer_size, uint64_t num_integers)
 	}
 
 	if (integer_size * num_integers > ZAP_MAXVALUELEN)
-		return (E2BIG);
+		return (SET_ERROR(E2BIG));
 
 	return (0);
 }