geom_flashmap: Rename the kernel module

Absent a linker.hints, if a module dependency exists on disk, the loader
will automatically load it. That is, if something depends on module
foo, and foo.ko exists, we'll load foo.ko even though the linker hints
file is missing. It's a bit of a hack but it's handy.

This breaks with geom_flashmap though, since it's geom_flashmap.ko on
disk but the module is called g_flashmap. However, pretty much every
other GEOM module is given a "geom_" prefix, so for consistency's sake
alone, it seems nice to rename the module.

PR: 274388
Reviewed by: jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47311