afs: Remote abort can cause BUG in rxrpc code

When writing files to afs I sometimes hit a BUG:

kernel BUG at fs/afs/rxrpc.c:179!

With a backtrace of:

afs_free_call
afs_make_call
afs_fs_store_data
afs_vnode_store_data
afs_write_back_from_locked_page
afs_writepages_region
afs_writepages

The cause is:

ASSERT(skb_queue_empty(&call->rx_queue));

Looking at a tcpdump of the session the abort happens because we
are exceeding our disk quota:

rx abort fs reply store-data error diskquota exceeded (32)

So the abort error is valid. We hit the BUG because we haven't
freed all the resources for the call.

By freeing any skbs in call->rx_queue before calling afs_free_call
we avoid hitting leaking memory and avoid hitting the BUG.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: <stable@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>