Inline zio_notify_parent() to reduce stack use

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

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

module/zfs/zio.c

@@ -435,7 +435,8 @@ zio_wait_for_children(zio_t *zio, enum zio_child child, enum zio_wait_type wait)
 	return (waiting);
 }
 
-static void
+__attribute__((always_inline))
+static inline void
 zio_notify_parent(zio_t *pio, zio_t *zio, enum zio_wait_type wait)
 {
 	uint64_t *countp = &pio->io_children[zio->io_child_type][wait];