xfs: don't mark all discard issued by zoned GC as sync

xfs: don't mark all discard issued by zoned GC as sync

Discard are not usually sync when issued from zoned garbage collection,
so drop the REQ_SYNC flag.

Fixes: 080d01c41d44 ("xfs: implement zoned garbage collection")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>