afs: Wait for outstanding async calls before closing rxrpc socket

The afs filesystem needs to wait for any outstanding asynchronous calls
(such as FS.GiveUpCallBacks cleaning up the callbacks lodged with a server)
to complete before closing the AF_RXRPC socket when unloading the module.

This may occur if the module is removed too quickly after unmounting all
filesystems.  This will produce an error report that looks like:

	AFS: Assertion failed
	1 == 0 is false
	0x1 == 0x0 is false
	------------[ cut here ]------------
	kernel BUG at ../fs/afs/rxrpc.c:135!
	...
	RIP: 0010:[<ffffffffa004111c>] afs_close_socket+0xec/0x107 [kafs]
	...
	Call Trace:
	 [<ffffffffa004a160>] afs_exit+0x1f/0x57 [kafs]
	 [<ffffffff810c30a0>] SyS_delete_module+0xec/0x17d
	 [<ffffffff81610417>] entry_SYSCALL_64_fastpath+0x12/0x6b

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>