btrfs: Deprecate BTRFS_SUBVOL_CREATE_ASYNC flag

Support for asynchronous snapshot creation was originally added in
72fd032e ("Btrfs: add SNAP_CREATE_ASYNC ioctl") to cater for
ceph's backend needs. However, since Ceph has deprecated support for
btrfs there is no longer need for that support in btrfs. Additionally,
this was never supported by btrfs-progs, the official userspace tools.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>