quota: Fix WARN_ON in lookup_one_len

We should hold i_mutex when looking up quota files for journaled quotas,
otherwise a WARN_ON in lookup_one_len triggers. The fact that we didn't
hold i_mutex previously probably could not lead to a real bug since the
filesystem is just being mounted / remounted read-write and thus the
root directory cannot change anyway but it's definitely cleaner with
i_mutex.

Reported-by: Bastien ROUCARIES <roucaries.bastien@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>