ipoib: Bound the number of egress mbufs buffered during pathrec lookups.

In pathological situations where the master subnet manager becomes
unresponsive for an extended period, we may otherwise end up queuing all
of the system's mbufs while waiting for a response to a path record lookup.

This addresses the same issue as commit 1e85b806f9 in Linux.

Reviewed by: cem, ngie
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division