Fix stack zio_execute()

Implement zio_execute() as a wrapper around the static function
__zio_execute() so that we can force  __zio_execute() to be inlined.
This reduces stack overhead which is important because __zio_execute()
is called recursively in several zio code paths.  zio_execute() itself
cannot be inlined because it is externally visible.

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