Fix taskq_wait_id()

The existing taskq_wait_id() function can incorrectly block
indefinitely.  Reimplement it more simply using wait_event()
in a similar fashion to taskq_wait_all().

This flaw was uncovered in the context of moving vn_rdwr() to
a taskq.  Previously taskq_wait_id() had no consumers outside
the SPLAT task framework which is why the issue went unnoticed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>