Unionfs: remove custom read/write methods

Unionfs: remove custom read/write methods

Having them results in lockdep warnings about having locks and grabbing the
same class locks in do_sync_read/write which were called from
unionfs_read/write.  All they did was revalidate out file object sooner,
which will now be deferred till a bit later.  Instead, use generic
do_sync_read and do_sync_write.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>