ksmbd: fix durable reconnect double-bind race in ksmbd_reopen_durable_fd

ksmbd: fix durable reconnect double-bind race in ksmbd_reopen_durable_fd

Two concurrent same-user DHnC reconnects can both observe fp->conn == NULL
before either sets it. ksmbd_reopen_durable_fd() checks fp->conn to guard
against a handle already being reconnected, but the check and the binding
assignment are not atomic: both threads pass the guard, both call
ksmbd_conn_get() on the same fp, and both eventually reach
kfree(fp->owner.name) -- a double-free of the owner.name slab object.
The double-bound ksmbd_file also causes a write-UAF on the 344-byte
ksmbd_file_cache object when a concurrent smb2_close() spins on fp->f_lock
after the object has been freed by the losing reconnect path.

KASAN on 7.1-rc5 (48-thread concurrent reconnect, 3000 cycles):
  BUG: KASAN: double-free in ksmbd_reopen_durable_fd+0x268/0x308
  BUG: KASAN: slab-use-after-free in _raw_spin_lock+0xac/0x150
    Write of size 4 at offset 24 into freed ksmbd_file_cache object
Five double-bind windows observed; 63 total KASAN reports triggered.

Fix: validate and claim fp->conn under write_lock(&global_ft.lock) so the
check-and-claim is atomic. ksmbd_lookup_durable_fd() already treats
fp->conn != NULL as "in use" and skips such an fp; setting fp->conn before
dropping the lock closes the race. ksmbd_conn_get() is a non-sleeping
refcount increment, safe under the rwlock. The rollback path on __open_id()
failure also clears fp->conn/tcon under the lock so concurrent readers see
a consistent state.

Fixes: b1f1e80620de ("ksmbd: centralize ksmbd_conn final release to plug transport leak")
Assisted-by: Henry (Claude):claude-opus-4
Signed-off-by: Gil Portnoy <dddhkts1@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>