dm kcopyd: reserve fewer pages

Reserve just the minimum of pages needed to process one job.

Because we allocate pages from page allocator, we don't need to reserve
a large number of pages.  The maximum job size is SUB_JOB_SIZE and we
calculate the number of reserved pages based on this.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

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

diff --cc drivers/md/dm-raid1.c
Simple merge

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

diff --cc include/linux/dm-kcopyd.h
Simple merge