afs: Make afs callbacks more flexible.

Currently we pass the information for callbacks (an int and a pointer
to an osl_object) as separate arguments. If additional information
must be passed to some callbacks, every callback must be modified to
match the new prototype, even those which won't use the new argument.

This commit introduces struct afs_callback_arg which contains the two
callback arguments and changes all callbacks to receive a pointer
to such a structure. This is an equivalent transformation with no
visible change in semantics.

With this commit in place it is easy to provide additional information
by simply extending struct afs_callback as appropriate.