Import P4 change 664228 from SpectraBSD.

Change 664228 by willa@willa_SpectraBSD on 2013/04/11 17:20:06

	Remove SPA_SET_ZIO_ERROR & ZIO_SET_ERROR for diff reduction.

	I added these macros in change 543543 nearly a year ago to make it
	easier to track down I/O errors inside the ZIO pipeline, since I/O
	errors sometimes have no obvious source.  They only did something
	other than set zio->io_error if you ran a ZFS_DEBUG build, so they
	don't benefit us in the field.

	However, Matt Ahrens wrote DTrace SDT probes for exactly the same
	purpose, and they were checked into illumos-gate a month ago:

	https://github.com/illumos/illumos-gate/commit/be6fd75a69ae679453d9cda5bff3326111e6d1ca

	Since this solution is superior, next time we need this functionality,
	we should simply port the above commit.