rxrpc: Lock around calling a kernel service Rx notification

Place a spinlock around the invocation of call->notify_rx() for a kernel
service call and lock again when ending the call and replace the
notification pointer with a pointer to a dummy function.

This is required because it's possible for rxrpc_notify_socket() to be
called after the call has been ended by the kernel service if called from
the asynchronous work function rxrpc_process_call().

However, rxrpc_notify_socket() currently only holds the RCU read lock when
invoking ->notify_rx(), which means that the afs_call struct would need to
be disposed of by call_rcu() rather than by kfree().

But we shouldn't see any notifications from a call after calling
rxrpc_kernel_end_call(), so a lock is required in rxrpc code.

Without this, we may see the call wait queue as having a corrupt spinlock:

    BUG: spinlock bad magic on CPU#0, kworker/0:2/1612
    general protection fault: 0000 [#1] SMP
    ...
    Workqueue: krxrpcd rxrpc_process_call
    task: ffff88040b83c400 task.stack: ffff88040adfc000
    RIP: 0010:spin_bug+0x161/0x18f
    RSP: 0018:ffff88040adffcc0 EFLAGS: 00010002
    RAX: 0000000000000032 RBX: 6b6b6b6b6b6b6b6b RCX: ffffffff81ab16cf
    RDX: ffff88041fa14c01 RSI: ffff88041fa0ccb8 RDI: ffff88041fa0ccb8
    RBP: ffff88040adffcd8 R08: 00000000ffffffff R09: 00000000ffffffff
    R10: ffff88040adffc60 R11: 000000000000022c R12: ffff88040aca2208
    R13: ffffffff81a58114 R14: 0000000000000000 R15: 0000000000000000
    ....
    Call Trace:
     do_raw_spin_lock+0x1d/0x89
     _raw_spin_lock_irqsave+0x3d/0x49
     ? __wake_up_common_lock+0x4c/0xa7
     __wake_up_common_lock+0x4c/0xa7
     ? __lock_is_held+0x47/0x7a
     __wake_up+0xe/0x10
     afs_wake_up_call_waiter+0x11b/0x122 [kafs]
     rxrpc_notify_socket+0x12b/0x258
     rxrpc_process_call+0x18e/0x7d0
     process_one_work+0x298/0x4de
     ? rescuer_thread+0x280/0x280
     worker_thread+0x1d1/0x2ae
     ? rescuer_thread+0x280/0x280
     kthread+0x12c/0x134
     ? kthread_create_on_node+0x3a/0x3a
     ret_from_fork+0x27/0x40

In this case, note the corrupt data in EBX.  The address of the offending
afs_call is in R12, plus the offset to the spinlock.

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