gfs2: Remove GFS2_MIN_LVB_SIZE define

The dlm lockspace is set up to have lock value blocks of GDLM_LVB_SIZE bytes,
and dlm is the only lock manager we support, so there is no point in claiming
that the lock value block could have any other size.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>