Unionfs: implement native unionfs_fsync/unionfs_fasync methods

Unionfs: implement native unionfs_fsync/unionfs_fasync methods

Unionfs is not a block-based file system, but it has to work with both
block-based file systems and non-block-based ones (i.e., when
CONFIG_BLOCK=n).  We used to define our ->fsync method to file_fsync, but
that's wrong because file_fsync partially depends on CONFIG_BLOCK=y.  And we
didn't define an fasync method: now we have both.  Moreover, at best,
file_fsync would have caused unionfs to sync up one lower branch---but as a
fanout file system, we need to sync up all valid lower branches.

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