Add unlocked/SMR fast path to getblk()

Convert the bufobj tries to an SMR zone/PCTRIE and add a gbincore_unlocked()
API wrapping this functionality. Use it for a fast path in getblkx(),
falling back to locked lookup if we raced a thread changing the buf's
identity.

Reported by: Attilio
Reviewed by: kib, markj
Testing: pho (in progress)
Sponsored by: Isilon
Differential Revision: https://reviews.freebsd.org/D25782