Improve performance of zio_taskq_member

__zio_execute() calls zio_taskq_member() to determine if we are running
in a zio interrupt taskq, in which case we may need to switch to
processing this zio in a zio issue taskq.  The call to
zio_taskq_member() can become a performance bottleneck when we are
processing a high rate of zio's.

zio_taskq_member() calls taskq_member() on each of the zio interrupt
taskqs, of which there are 21.  This is slow because each call to
taskq_member() does tsd_get(taskq_tsd), which on Linux is relatively
slow.

This commit improves the performance of zio_taskq_member() by having it
cache the value of tsd_get(taskq_tsd), reducing the number of those
calls to 1/21th of the current behavior.

In a test case running `zfs send -c >/dev/null` of a filesystem with
small blocks (average 2.5KB/block), zio_taskq_member() was using 6.7% of
one CPU, and with this change it is reduced to 1.3%.  Overall time to
perform the `zfs send` reduced by 10% (~150,000 block/sec to ~165,000
blocks/sec).

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Serapheim Dimitropoulos <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Tony Nguyen <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #10070 

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