[ppc64] llan: fix fatal kernel trap when system is low on memory

When running several builders in parallel, on QEMU, with 8GB of
memory, a fatal kernel trap (0x300 (data storage interrupt))
caused by llan driver is sometimes observed, when the system
starts to run out of swap space.

This happens because, at llan_intr(), a phyp call to add a
logical LAN buffer is always made when llan_add_rxbuf() fails,
even if it fails to allocate a new buffer.

PR: 235489
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D19084