gfs2: only apply DLM_LKF_VALBLK if sb_lvbptr is not NULL

Currently, gfs2 always sets the DLM_LKF_VALBLK flag to enable lvb
handling even when sb_lvbptr is NULL.  This currently causes no problems
because DLM ignores the DLM_LKF_VALBLK flag when sb_lvbptr is NULL, but
it does violate the DLM API.  Fix that by only setting DLM_LKF_VALBLK
when sb_lvbptr is not NULL.

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