generic: test journal/log replay when a file with cloned extents was fsync'ed

generic: test journal/log replay when a file with cloned extents was fsync'ed

Test that if we clone part of an extent from a file to itself at a
different offset, fsync it, rewrite (COW) part of the extent from the
former offset, fsync it again, power fail and then mount the filesystem,
we are able to read the whole file and it has the correct data.

This is motivated by a bug found in btrfs which is fixed by a kernel patch
that has the following subject:

  "Btrfs: fix missing data checksums after replaying a log tree"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>