udf: cleanup directory offset handling

Position in directory returned by readdir is offset of directory entry divided
by four (don't ask me why).  Make this conversion only when reading f_pos from
userspace / writing it there and internally work in bytes.  It makes things
more easily readable and also fixes a bug (we forgot to divide length of the
entry by 4 when advancing f_pos in udf_add_entry()).

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>