dm thin: display 'needs_check' in status if it is set

There is currently no way to see that the needs_check flag has been set
in the metadata.  Display 'needs_check' in the thin-pool status if it is
set in the thinp metadata.

Also, update thinp documentation.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --cc Documentation/device-mapper/thin-provisioning.txt
Simple merge

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