ext4: check for negative error code from sb_issue_discard

sb_issue_discard() is returning negative error code, so check for
-EOPNOTSUPP.
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -2613,7 +2613,7 @@ static inline void ext4_issue_discard(struct super_block *sb,
 	trace_ext4_discard_blocks(sb,
 			(unsigned long long) discard_block, count);
 	ret = sb_issue_discard(sb, discard_block, count);
-	if (ret == EOPNOTSUPP) {
+	if (ret == -EOPNOTSUPP) {
 		ext4_warning(sb, "discard not supported, disabling");
 		clear_opt(EXT4_SB(sb)->s_mount_opt, DISCARD);
 	}