ksmbd: fix durable fd leak on ClientGUID mismatch in durable v2 open

ksmbd_lookup_fd_cguid() returns a ksmbd_file with its refcount
incremented via ksmbd_fp_get(). parse_durable_handle_context() in
the DURABLE_REQ_V2 case properly releases this reference on every
path inside the ClientGUID-match branch, either by calling
ksmbd_put_durable_fd() or by transferring ownership to dh_info->fp
for a successful reconnect. However, when an entry exists in the
global file table with the same CreateGuid but a different
ClientGUID, the code simply falls through to the new-open path
without dropping the reference obtained from ksmbd_lookup_fd_cguid().

Per MS-SMB2 section 3.3.5.9.10 ("Handling the
SMB2_CREATE_DURABLE_HANDLE_REQUEST_V2 Create Context"), the server
MUST locate an Open whose Open.CreateGuid matches the request's
CreateGuid AND whose Open.ClientGuid matches the ClientGuid of the
connection that received the request. If no such Open is found, the
server MUST continue with the normal open execution phase. A
CreateGuid hit with a ClientGUID mismatch is therefore the
"Open not found" case: proceeding with a new open is correct, but
the reference obtained purely as a side effect of the lookup must
not be leaked.

Repeated requests that hit this mismatch pin global_ft entries,
prevent __ksmbd_close_fd() from ever running for the corresponding
files, and defeat the durable scavenger, leading to long-lived
resource leaks.

Release the reference in the mismatch path and clear dh_info->fp so
subsequent logic does not mistake a non-matching lookup result for
a reconnect target.

Fixes: c8efcc786146 ("ksmbd: add support for durable handles v1/v2")
Signed-off-by: DaeMyung Kang <charsyam@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>