misc_scan: don't trust the mapping file too much for device names.

misc_scan: don't trust the mapping file too much for device names.

misc_scan assumes that any device name found in the 'mapping' file
is usable.  Usually it is but sometimes not, such as for inactive
devices.
Depending on it isn't really robust, when a name is found, check that
it exists. If not, fall back on map_dev.

This will allow "--detail --scan" to notice inactive devices.

Signed-off-by: NeilBrown <neilb@suse.de>