ext4: add missing unlock to an error path in ext4_quota_write()

When write in ext4_quota_write() fails, we have to properly release
i_mutex.  One error path has been missing the unlock...

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>