ext4: release discard bio after sending discard commands

We've changed the discard command handling into parallel manner.
But, in this change, I forgot decreasing the usage count of the bio
which was used to send discard request. I'm sorry about that.

Fixes: a0154344 ("ext4: send parallel discards on commit completions")
Signed-off-by: Daeho Jeong <daeho.jeong@samsung.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/ext4/mballoc.c

@@ -2892,8 +2892,10 @@ void ext4_process_freed_data(struct super_block *sb, tid_t commit_tid)
 				break;
 		}
 
-		if (discard_bio)
+		if (discard_bio) {
 			submit_bio_wait(discard_bio);
+			bio_put(discard_bio);
+		}
 	}
 
 	list_for_each_entry_safe(entry, tmp, &freed_data_list, efd_list)