Add TASKQID_INVALID and TASKQID_INITIAL macros

Add TASKQID_INVALID and TASKQID_INITIAL macros

Add the TASKQID_INVALID and TASKQID_INITIAL macros and update the
taskq implementation and test cases to use them.  This is solely
for the purposes of readability and introduces no functional change.

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