Inline zio_execute() to reduce stack use

Deep recursive call chains are contributing to segfaults in ztest due to
heavy stack use.  Inlining zio_execute() helps reduce the stack depth of
the zio_notify_parent() -> zio_execute() -> zio_wait() recursive cycle.
I am no longer seeing ztest segfaults in this code path with this change.

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

module/zfs/zio.c

@@ -1119,7 +1119,8 @@ zio_interrupt(zio_t *zio)
  */
 static zio_pipe_stage_t *zio_pipeline[];
 
-void
+__attribute__((always_inline))
+inline void
 zio_execute(zio_t *zio)
 {
 	zio->io_executor = curthread;