generic: test fsync after moving files across directories

generic: test fsync after moving files across directories

Test file A fsync after moving one other unrelated file B between
directories and fsyncing B's old parent directory before fsyncing
the file A. Check that after a crash all the file A data we fsynced
is available.

This test is motivated by an issue discovered in btrfs which caused
the file data to be lost (despite fsync returning success to user
space). That btrfs bug is fixed by the following linux kernel patch:

   Btrfs: fix data loss in the fast fsync path

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>