btrfs: test that fiemap reports extent as not shared after deleting file

btrfs: test that fiemap reports extent as not shared after deleting file

Test that if we have two files with shared extents, after removing one of
the files, if we do a fiemap against the other file, it does not report
extents as shared anymore.

This exercises the processing of delayed references for data extents in
the backref walking code, used by fiemap to determine if an extent is
shared.

This used to fail until very recently and was fixed by the following
kernel commit that landed in 6.1-rc2:

  4fc7b5722824 (""btrfs: fix processing of delayed data refs during backref walking")

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