afs: Fix afs_fs_fetch_data() to set call->async

afs: Fix afs_fs_fetch_data() to set call->async

Fix afs_fs_fetch_data() to set call->async on an async operation as does
afs_fs_fetch_data64().

Fixes: eddf51f2bb2c ("afs: Make {Y,}FS.FetchData an asynchronous operation")
Link: https://sashiko.dev/#/patchset/20260702144919.172295-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260723113452.566619-2-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>