GFS2: Initialize hex string to '0'

When generating the DLM lock name, a value of 0 would skip
the loop and leave the string unchanged.  This left locks with
a value of 0 unlabeled.  Initializing the string to '0' fixes this.

Signed-off-by: Nathan Straz <nstraz@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>