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

In the Linux kernel, the following vulnerability has been resolved: 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.