ksmbd: fix kernel oops from idr_remove()

There is a report that kernel oops happen from idr_remove().

kernel: BUG: kernel NULL pointer dereference, address: 0000000000000010
kernel: RIP: 0010:idr_remove+0x1/0x20
kernel:  __ksmbd_close_fd+0xb2/0x2d0 [ksmbd]
kernel:  ksmbd_vfs_read+0x91/0x190 [ksmbd]
kernel:  ksmbd_fd_put+0x29/0x40 [ksmbd]
kernel:  smb2_read+0x210/0x390 [ksmbd]
kernel:  __process_request+0xa4/0x180 [ksmbd]
kernel:  __handle_ksmbd_work+0xf0/0x290 [ksmbd]
kernel:  handle_ksmbd_work+0x2d/0x50 [ksmbd]
kernel:  process_one_work+0x21d/0x3f0
kernel:  worker_thread+0x50/0x3d0
kernel:  rescuer_thread+0x390/0x390
kernel:  kthread+0xee/0x120
kthread_complete_and_exit+0x20/0x20
kernel:  ret_from_fork+0x22/0x30

While accessing files, If connection is disconnected, windows send
session setup request included previous session destroy. But while still
processing requests on previous session, this request destroy file
table, which mean file table idr will be freed and set to NULL.
So kernel oops happen from ft->idr in __ksmbd_close_fd().
This patch don't directly destroy session in destroy_previous_session().
It just set to KSMBD_SESS_EXITING so that connection will be
terminated after finishing the rest of requests.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>