Add kstat entries for ZFS compression statistics.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/sys/zio_compress.h:
sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zio_compress.c:
	Add module lifetime functions to allocate and teardown
	state data.

	Report:
		- Compression attempts.
		- Buffers found to be empty.
		- Compression calls that are skipped because
		  the data length is already less than or
		  equal to the minimum block length.
		- Compression attempts that fail to yield a 12.5%
		  compression ratio.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu.c:
	Add calls to the zio_compress.c module's init and fini
	functions.

Sponosred by:	Spectra Logic Corporation
MFC after:	2 weeks