During vdev_geom_open, require that the vdev guids match the device's label
except during split, add, or create operations. This fixes a bug where the
wrong disk could be returned, and higher layers of ZFS would immediately
eject it again.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c:
o When opening by GUID, require both the pool and vdev GUIDs to

	  match.  While it is highly unlikely for two vdevs to have the same
	  vdev GUIDs, the ZFS storage pool allocator only guarantees they
	  are unique within a pool.

o Modify the open behavior to:

• If we are opening a vdev that hasn't previously been opened,

	    open by path without checking GUIDs.
	  - Otherwise, open by path and verify GUIDs.
	  - If that fails, search all geom providers for a device with
	    matching GUIDs.
	  - If that fails, return ENOENT.

Submitted by: gibbs, asomers
Reviewed by: smh
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D4486