afs: Split wait from afs_make_call()

Split the call to afs_wait_for_call_to_complete() from afs_make_call() to
make it easier to handle asynchronous calls and to make it easier to
convert a synchronous call to an asynchronous one in future, for instance
when someone tries to interrupt an operation by pressing Ctrl-C.

Signed-off-by: David Howells <dhowells@redhat.com>