generic: test fiemap reports extent as shared after cloning it

generic: test fiemap reports extent as shared after cloning it

Test that if we have two consecutive extents and only one of them is
cloned, then fiemap correctly reports which one is shared and reports
the other as not shared.

This currently fails on btrfs for all kernel releases, but is fixed by
a kernel patch that landed in Linus' tree last week:

  ac3c0d36a2a2f7 ("btrfs: make fiemap more efficient and accurate reporting extent sharedness")

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