efivar: Really look for labels for the provider with right efimedia

The prior code mistakently thought that the g_consumer that hung off the
provider we found were the right thing to use to find all the glabel
aliases for this node. However, the only way to find that is to iterate
through all the geoms that belong to the glabel geom class, looking for
those geoms with the same name as the provider with the right efimedia.
Do this in a way that caches glabel class, and allows for it to be
absent. Tighten the filter for mounted filesystems to only look
for the ones that are mounted on /dev/.. since the rest of the code
assumes that.

MFC After: 3 days
Sponsored by: Netflix
Reviewed by: corvink, asomers
Differential Revision: https://reviews.freebsd.org/D38619