zfs: convert z_teardown_inactive_lock to sleepable read-mostly lock

This eliminates a global serialisation point. It only gets write locked
on unmount.

Sample result doing an incremental -j 40 build:
before: 173.30s user 458.97s system 2595% cpu 24.358 total
after: 168.58s user 254.92s system 2211% cpu 19.147 total