iflib: Fix typo in 'iflib_rx_miss_bufs' sysctl name

It looks like these sysctls were copy-pasted from netmap. Most were changed
from 'ixl_' prefix to 'iflib_', but this one was missed.

Fix the "can't re-use a leaf (ixl_rx_miss_bufs)!" warning.

Reported by: dim@ and others
Sponsored by: EMC / Isilon Storage Division