Revert "Fix __zio_execute() asynchronous dispatch"

This reverts commit 91579709fc which
limited the asynchronous dispatch to kernel space.  We want to do
this for two reasons:

1) While we have slightly more headroom in user space excessively
   deep stacks have been observed while running ztest, see #2293.

2) Removing this conditional makes the pipeline behave consistently
   regardless of if it's executing in kernel space or user space.
   This way we're more likely to uncover subtle issues with ztest.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2384

module/zfs/zio.c

@@ -1342,7 +1342,6 @@ __zio_execute(zio_t *zio)
 			return;
 		}
 
-#ifdef _KERNEL
 		/*
 		 * If we executing in the context of the tx_sync_thread,
 		 * or we are performing pool initialization outside of a
@@ -1357,7 +1356,6 @@ __zio_execute(zio_t *zio)
 			zio_taskq_dispatch(zio, ZIO_TASKQ_ISSUE, cut);
 			return;
 		}
-#endif
 
 		zio->io_stage = stage;
 		rv = zio_pipeline[highbit(stage) - 1](zio);