Remove suffix from zio taskq names

Remove suffix from zio taskq names

For zio taskq's which have multiple instances (e.g. z_rd_int_0,
z_rd_int_1, etc), each one has a unique name (the _0, _1, _2 suffix).
This makes performance analysis more difficult, because by default,
`perf` includes the thread name (which is the same as the taskq name) in
the stack trace.  This means that we get 8 different stacks, all of
which are doing the same thing, but are executed from different taskq's.

We should remove the suffix of the taskq name, so that all the
read-interrupt threads are named z_rd_int.

Note that we already support multiple taskq's with the same name.  This
happens when there are multiple pools.  In this case the taskq has a
different tq_instance, which shows up in /proc/spl/taskq-all.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #7646