udf: Use i_size_read() in udf_adinicb_writepage()

We don't hold inode_lock in udf_adinicb_writepage() so use i_size_read()
to get i_size. This cannot cause real problems is i_size is guaranteed
to be small but let's be careful.

Signed-off-by: Jan Kara <jack@suse.cz>