migration/multifd: Fix compat with QEMU < 9.0

migration/multifd: Fix compat with QEMU < 9.0

Commit f5f48a7891 ("migration/multifd: Separate SYNC request with
normal jobs") changed the multifd source side to stop sending data
along with the MULTIFD_FLAG_SYNC, effectively introducing the concept
of a SYNC-only packet. Relying on that, commit d7e58f412c
("migration/multifd: Don't send ram data during SYNC") later came
along and skipped reading data from SYNC packets.

In a versions timeline like this:

  8.2 f5f48a7 9.0 9.1 d7e58f41 9.2

The issue arises that QEMUs < 9.0 still send data along with SYNC, but
QEMUs > 9.1 don't gather that data anymore. This leads to various
kinds of migration failures due to desync/missing data.

Stop checking for a SYNC packet on the destination and unconditionally
unfill the packet.

>From now on:

old -> new:
the source sends data + sync, destination reads normally

new -> new:
source sends only sync, destination reads zeros

new -> old:
source sends only sync, destination reads zeros

CC: qemu-stable@nongnu.org
Fixes: d7e58f412c ("migration/multifd: Don't send ram data during SYNC")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2720
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20241213160120.23880-2-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit b93d897ea2f0abbe7fc341a9ac176b5ecd0f3c93)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>