rxrpc, afs: Fix missing error pointer check after rxrpc_kernel_lookup_peer()

rxrpc, afs: Fix missing error pointer check after rxrpc_kernel_lookup_peer()

rxrpc_kernel_lookup_peer() can also return error pointers in addition to
NULL, so just checking for NULL is not sufficient.

Fix this by:

 (1) Changing rxrpc_kernel_lookup_peer() to return -ENOMEM rather than NULL
     on allocation failure.

 (2) Making the callers in afs use IS_ERR() and PTR_ERR() to pass on the
     error code returned.

Fixes: 72904d7b9bfb ("rxrpc, afs: Allow afs to pin rxrpc_peer objects")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Co-developed-by: David Howells <dhowells@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/368272.1772713861@warthog.procyon.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>