multipath: better argument type checking

multipath: better argument type checking

The way that multipath decided what you passed in as an argument didn't
always work.  If the argument was the name of a file, then multipath
assumed that it was a path.  That meant if you were in /dev/mapper and ran

# multipath -f <mpath_device_name>

It would fail, since it thought you gave it a path name, instead of a
multipath device name.  Now multipath will only treat the argument
as a path name if it is a block device with a different major number than
device-mapper's.  Also, I've switched the MAJOR/MINOR/MKDEV macros to
work like kpartx, so that they can handle minor numbers over 255.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>