dm table: add immutable feature

Introduce DM_TARGET_IMMUTABLE to indicate that the target type cannot be mixed
with any other target type, and once loaded into a device, it cannot be
replaced with a table containing a different type.

The thin provisioning pool device will use this.

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

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

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

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

diff --cc drivers/md/dm.h
Simple merge

diff --cc include/linux/device-mapper.h
Simple merge

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