btrfs: send: use fallocate for hole punching with send stream v2

Currently holes are sent as writes full of zeroes, which results in
unnecessarily using disk space at the receiving end and increasing the
stream size.

In some cases we avoid sending writes of zeroes, like during a full
send operation where we just skip writes for holes.

But for some cases we fill previous holes with writes of zeroes too, like
in this scenario:

1) We have a file with a hole in the range [2M, 3M), we snapshot the
   subvolume and do a full send. The range [2M, 3M) stays as a hole at
   the receiver since we skip sending write commands full of zeroes;

2) We punch a hole for the range [3M, 4M) in our file, so that now it
   has a 2M hole in the range [2M, 4M), and snapshot the subvolume.
   Now if we do an incremental send, we will send write commands full
   of zeroes for the range [2M, 4M), removing the hole for [2M, 3M) at
   the receiver.

We could improve cases such as this last one by doing additional
comparisons of file extent items (or their absence) between the parent
and send snapshots, but that's a lot of code to add plus additional CPU
and IO costs.

Since the send stream v2 already has a fallocate command and btrfs-progs
implements a callback to execute fallocate since the send stream v2
support was added to it, update the kernel to use fallocate for punching
holes for V2+ streams.

Test coverage is provided by btrfs/284 which is a version of btrfs/007
that exercises send stream v2 instead of v1, using fsstress with random
operations and fssum to verify file contents.

Link: https://github.com/kdave/btrfs-progs/issues/1001


CC: stable@vger.kernel.org # 6.1+
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>