Because destroy_all_dev() checks the mask/match against the device's si_udev,

Because destroy_all_dev() checks the mask/match against the device's si_udev,
which is a field combining both major and minor numbers, we must mask off the
major bits (to retain only the minor bits) from the 'mask' variable in order
to allow a generic -1 to be passed as the mask.  Otherwise we will not match
anything.

Revamp a good chunk of the documentation to try to make the major/minor
number masking issues clear.

Bug-found-by: Chuck Tuffli <chuck_tuffli@agilent.com>