Make sure the so-called end of receive interrupts don't starve in iflib.

When the receive ring cannot be filled with mbufs, due to lack of memory,
no more interrupts may be generated to fill the receive ring later on.
Make sure to have a watchdog, to try refilling the receive ring from time
to time, hopefully when more mbufs are available.

Differential Revision: https://reviews.freebsd.org/D23315
MFC after: 1 week
Reviewed by: gallatin@
Sponsored by: Mellanox Technologies