ipfw: do not use errno value for error reporting

table_do_modify_record() already uses errno value on error.
Also this fixes problem when `ipfw table add` returns ENOTTY that
is unrelated to operation.

Tested by:	dhw
Fixes:		09025a7147
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D53050

sbin/ipfw/tables.c

@@ -1037,9 +1037,6 @@ table_modify_record(ipfw_obj_header *oh, int ac, char *av[], int add,
 		}
 	}
 
-	/* Get real OS error */
-	error = errno;
-
 	/* Report results back */
 	ptent = tent_buf;
 	for (i = 0; i < count; ptent++, i++) {