fs: dlm: make dlm sequence id more robust

When joining a new lockspace, use a random number to initialize
a sequence number used in messages. This makes it easier to detect
sequence number mismatches in message replies during tests that
repeatedly join and leave a lockspace.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>