xnbd_sysfs: Take global session list mutex when adding/removing devices

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Rename drive_list -> devs_list

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Protect session devices using a spinlock

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Remove redundant variables

- global files list
- file lock

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Make xnbd_destroy_device static void

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Make xnbd_destroy_session_devices static void

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
xnbd: Add support for remove host via sysfs

Signed-off-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>