Previously, this didn't work because L2ARC devices' labels don't contain
pool GUIDs. Modify zfsd so that the pool GUID won't be required:
lib/libdevdctl/guid.h
Change INVALID_GUID from a uint64_t constant to a function that
returns an invalid Guid object. Remove the void constructor.
Nothing uses it, and it violates RAII.
cddl/usr.sbin/zfsd/case_file.h
cddl/usr.sbin/zfsd/case_file.cc
Allow CaseFile::Find to match a CaseFile based on Vdev GUID alone.
In CaseFile::ReEvaluate, attempt to online devices even if the newly
arrived device has no pool GUID.
cddl/usr.sbin/zfsd/vdev_iterator.cc
Iterate through a pool's cache devices as well as its regular
devices.
Reported by: avg
Reviewed by: avg
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D12791
12a88a3d63