The amount of memory allocated in nbd_co_receive_request is driven by the
NBD client (possibly a virtual machine).  Parallel I/O can cause the
server to allocate a large amount of memory; check for failures and
return ENOMEM in that case.

Cc: qemu-block@nongnu.org
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>