dm table: add always writeable feature

Add a target feature flag DM_TARGET_ALWAYS_WRITEABLE to indicate that a target
does not support read-only mode.

The initial implementation of the thin provisioning target uses this.

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

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

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