intrng: track counter allocation with a bitmap

Crucially, this allows releasing counters, and interrupt sources by
extension. Where before we were incrementing intrcnt_index with atomics,
now we protect the bitmap using the existing isrc_table_lock mutex.

Reviewed by: mmel
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D38437

(cherry picked from commit 28137bdb19aa34b8351108de4257795a93c0ba09)