alc: bounds-check the Rx return descriptor index

[ojanerif_amd.com]

Summary

alc_rxeof() used the 12-bit RD_IDX field from the Rx return descriptor
to index the 256-entry Rx descriptor ring without validation; the first
access could fall outside the ring. Drop the frame if the index is
outside the ring.

[markj]

sys/dev/alc/if_alc.c

3640

We should also increment IFCOUNTER_IERRORS here.

[ojanerif_amd.com]

Good catch. Added if_inc_counter(ifp, IFCOUNTER_IERRORS, 1) in the reject branch, matching the idiom used for malformed Rx descriptors in sge_rxeof() (sys/dev/sge/if_sge.c). ifp was already in scope there.