devmapper: Move maxDeviceID check in loadMetadata

maxDeviceID is upper limit on device Id thin pool can support. Right now
we have this check only during startup. It is a good idea to move this
check in loadMetadata so that any time a device file is loaded and if it
is corrupted and device Id is more than maxDevieceID, it will be detected
right then and there.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>