Refactor vdev_geom_attach and friends to reduce code duplication

Refactor vdev_geom_attach and friends to reduce code duplication

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
	Move checks for provider's sectorsize and mediasize into a single
	location in vdev_geom_attach. Remove the zfs::vdev::taste class;
	it's ok to use the regular vdev class for tasting. Consolidate guid
	checks into a single location in vdev_attach_ok. Consolidate some
	error handling code from vdev_geom_attach into vdev_geom_detach,
	closing a resource leak of geom consumers in the process.

Reviewed by:	avg
MFC after:	4 weeks
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D5974