Hit following build failure on b2_12, we've applied LU-12635 patches, but this looks like a leftover from that ticket.

/lnet/lnet/net_fault.c:689:5: error: this statement may fall through [-Werror=implicit-fallthrough=]
lnet_ni_recv(ni, msg->msg_private, msg, 0,
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0, msg->msg_len, msg->msg_len);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/lnet/lnet/net_fault.c:691:4: note: here
case LNET_CREDIT_WAIT:
^~~~