Change KM_SLEEP to TQ_SLEEP in spa_deadman()

Since they both evaluate to zero, this is a semi-cosmetic change
but the latter is the proper value to use as an argument to
taskq_dispatch_delay().

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4393