mon/FSCommands: Fix 'add_data_pool' command

mon/FSCommands: Fix 'add_data_pool' command

After making a RADOS pool a filesystem's data pool using the
'add_data_pool' command, the value of the 'data' key of the pool's
application metadata 'cephfs' should be the filesystem's name. This
didn't happen when the pool's application metadata 'cephfs' was
enabled before the pool was made the data pool. Fix this during the
handling of the 'add_data_pool' command by setting the value of
the 'data' key of the pool's application metadata 'cephfs' to the
filesystem's name even when the application metadata 'cephfs' is
already enabled or set.

Fixes: https://tracker.ceph.com/issues/43061
Signed-off-by: Ramana Raja <rraja@redhat.com>
(cherry picked from commit 3c49092322720b7f1d612f45e73fb26820ff7885)