nvmet: add safety check for subsys lock

Replace comment about required lock with a lockdep_assert_held()
check in the following functions:
 - nvmet_p2pmem_ns_add_p2p()
 - nvmet_setup_p2p_ns_map()
 - nvmet_release_p2p_ns_map()

This ensures the subsystem lock is held at runtime.

Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>