nvmet: fix error flow during ns enable

In case we fail to enable p2pmem on the current namespace, disable the
backing store device before exiting.

Cc: Stephen Bates <sbates@raithlin.com>
Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>