udf: fix adding entry to a directory

When adding directory entry to a directory, we have to properly increase
length of the last extent.  Handle this similarly as extending regular files -
make extents always have size multiple of block size (it will be truncated
down to proper size in udf_clear_inode()).

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>