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 <behlendorf1@llnl.gov>
Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10070

include/os/linux/spl/sys/taskq.h

@@ -151,6 +151,7 @@ extern void taskq_wait_outstanding(taskq_t *, taskqid_t);
 extern void taskq_wait(taskq_t *);
 extern int taskq_cancel_id(taskq_t *, taskqid_t);
 extern int taskq_member(taskq_t *, kthread_t *);
+extern taskq_t *taskq_of_curthread(void);
 
 #define	taskq_create_proc(name, nthreads, pri, min, max, proc, flags) \
     taskq_create(name, nthreads, pri, min, max, flags)