g_provider_by_name(9): argument can be a geom name or fullpath

The argument passed to g_provider_by_name(9) can be a geom name or a
fullpath.

• g_provider_by_name() gained this functionality in 769afdc71ea6b0c09dae883ce49d913cbc8cff5b.

Reviewed by: imp, kevans
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D27566