dm thin: eliminate the no_free_space flag

The pool's error_if_no_space flag can easily serve the same purpose that
no_free_space did, namely: control whether handle_unserviceable_bio()
will error a bio or requeue it.

This is cleaner since error_if_no_space is established when the pool's
features are processed during table load.  So it avoids managing the
no_free_space flag by taking the pool's spinlock.

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