nvme-loop: fix nvme-loop Kconfig dependencies

I ran into the same problem on NVME_TARGET_RDMA now,
which otherwise needs dependencies on both CONFIG_BLOCK and
CONFIGFS_FS:

    warning: (NVME_TARGET_LOOP && NVME_TARGET_RDMA) selects NVME_TARGET which has unmet direct dependencies (BLOCK && CONFIGFS_FS)
    0xA002B368 Mon Jul 11 18:00:45 CEST 2016 failed
    In file included from ../drivers/nvme/target/core.c:16:0:
    drivers/nvme/target/nvmet.h:222:14: error: field 'inline_bio' has incomplete type
      struct bio  inline_bio;
                  ^~~~~~~~~~
    drivers/nvme/target/core.c: In function 'nvmet_async_event_work':
    drivers/nvme/target/core.c:98:3: error: implicit declaration of function 'kfree' [-Werror=implicit-function-declaration]
       kfree(aen);
       ^~~~~
    ../drivers/nvme/target/core.c: In function 'nvmet_ns_enable':
    ../drivers/nvme/target/core.c:269:13: error: implicit declaration of function 'blkdev_get_by_path' [-Werror=implicit-function-declaration]
      ns->bdev = blkdev_get_by_path(ns->device_path, FMODE_READ | FMODE_WRITE,

Folding in my patch below should address that too.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>