Upstream: zil_commit_waiter() can stall forever

On macOS clock_t is unsigned, so when cv_timedwait_hires() returns -1
we loop forever. The conditional was tweaked to ignore signedness.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #10445