udf: Return error from udf_find_entry()

Return appropriate error from udf_find_entry() instead of just NULL.
That way we can distinguish the fact that some error happened when
looking up filename (and return error to userspace) from the fact that
we just didn't find the filename. Also update callers of
udf_find_entry() accordingly.

[JK: Improved udf_find_entry() documentation]

Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jan Kara <jack@suse.cz>