dm thin: allow metadata commit if pool is in PM_OUT_OF_DATA_SPACE mode

Commit 3e1a0699 ("dm thin: fix out of data space handling") introduced
a regression in the metadata commit() method by returning an error if
the pool is in PM_OUT_OF_DATA_SPACE mode.  This oversight caused a thin
device to return errors even if the default queue_if_no_space ENOSPC
handling mode is used.

Fix commit() to only fail if pool is in PM_READ_ONLY or PM_FAIL mode.

Reported-by:  <qindehua@163.com>
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org # 3.14+