generic: test swap activation on file that used to have clones

generic: test swap activation on file that used to have clones

Test that we are able to activate a swap file on a file that used to have
its extents shared multiple times.

This exercises a bug on btrfs' extent sharedness detection during swap
file activation, which is fixed by the following kernel commit:

  03018e5d8508 ("btrfs: fix swap file activation failure due to extents that used to be shared")

The fails sporadically on XFS and the bug was already reported to the XFS
mailing list:

   https://lore.kernel.org/linux-xfs/CAL3q7H7cURmnkJfUUx44HM3q=xKmqHb80eRdisErD_x8rU4+0Q@mail.gmail.com/

   https://lore.kernel.org/fstests/dca49a16a7aacdab831b8895bdecbbb52c0e609c.1733928765.git.fdmanana@suse.com/

So for now skip the test on XFS and add comments with references to these
threads.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>