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Support device-independent labels for geom_flashmap slices.

Description

Support device-independent labels for geom_flashmap slices.

While geom_flashmap has always supported label names for its slices, it does
so by appending "s.labelname" to the provider device name, meaning you still
have to know the name and unit of the hardware device to use the labels.

These changes add support for device-independent geom_flashmap labels, using
the standard geom_label infrastructure. geom_flashmap now creates a softc
struct attached to its geom, and as it creates slices it stores the label
into an array in the softc. The new geom_label_flashmap uses those labels
when tasting a geom_flashmap provider.

Differential Revision: https://reviews.freebsd.org/D19535