fs: dlm: stop unnecessarily filling zero ms_extra bytes

Commit 7175e131 ("fs: dlm: fix invalid derefence of sb_lvbptr")
fixes an issue when the lkb->lkb_lvbptr set to an dangled pointer and an
followed memcpy() would fail. It was fixed by an additional check of
DLM_LKF_VALBLK flag. The mentioned commit forgot to add an additional check
if DLM_LKF_VALBLK is set for the additional amount of LVB data allocated
in a dlm message. This patch is changing the message allocation to check
additionally if DLM_LKF_VALBLK is set otherwise a dangled lkb->lkb_lvbptr
pointer would allocated zero LVB message data which not gets filled with
actual data.

This patch is however only a cleanup to reduce the amount of zero bytes
transmitted over network as receive_lvb() will only evaluates message LVB
data if DLM_LKF_VALBLK is set.

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