AFS: Pass an afs_call* to call->async_workfn() instead of a work_struct*

call->async_workfn() can take an afs_call* arg rather than a work_struct* as
the functions assigned there are now called from afs_async_workfn() which has
to call container_of() anyway.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Nathaniel Wesley Filardo <nwf@cs.jhu.edu>
Reviewed-by: Tejun Heo <tj@kernel.org>