fs: ubifs: rely on higher layer to do unaligned read

Currently ubifs doesn't support unaligned read offset, thanks to the
recent _fs_read() work to handle unaligned read, we only need to
implement ubifs_get_blocksize() to take advantage of it.

Now ubifs can do unaligned read without any problem.

Signed-off-by: Qu Wenruo <wqu@suse.com>