[GFS2] lockproto api prefix

Use the gfs2_ prefix on the register/unregister functions for the lock
modules.  The gfs_ prefix was left from an old idea on how to share these
with gfs1.

Signed-off-by: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>