Refactor vdev_geom_attach and friends to reduce code duplication

[asomers]

Summary

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.

Test Plan

ZFS test suite

[asomers]

I declare a timeout on the CR. But by all means feel free to criticize it post-commit.

[avg]

In general I like this change very much as the code becomes safer and more readable.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c

285 ↗

(On Diff #15253)

Could it happen that the acr was incremented by somebody else and we really shouldn't decrement the access bits?
I would be more comfortable if we passed an explicit flag to this function which would instruct whether we should "unaccess" rather than trying to guess that.

293 ↗

(On Diff #15253)

Just curious, in what case this condition could be false?

[asomers]

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c

285 ↗

(On Diff #15253)

I'll do it.

293 ↗

(On Diff #15253)

Uh, never? Looks like this line was copied by mistake.

[asomers]

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c

318 ↗

(On Diff #15253)

This assertion isn't always true in the case of spare vdevs, where two vdevs can share a consumer. I'll remove it. When I commit the change that turns cp->private into a linked list I'll restore a similar assertion.

[asomers]

Apply avg's comments. Remove a tautological assertion, remove an incorrect
assertion, and fix some theoretical null pointer dereferences in
vdev_geom_detach when vfs.zfs.debug=1.