Revert "Inline zio_execute() to reduce stack use"

This reverts commit 411dd65af1.

gcc version 4.1.2 does not like having differing prototypes
for zio_execute, one version in the .c with inline and one
version in the .h without.  Thus I'm reverting this change
and we'll see how critical this particular stack reduction is.

module/zfs/zio.c

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