dm: use vzalloc

Use vzalloc() instead of vmalloc()+memset().

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --cc drivers/md/dm-log.c
Simple merge

diff --cc drivers/md/dm-snap-persistent.c
Simple merge

diff --cc drivers/md/dm-table.c
Simple merge