fs: cifs: remove unneeded variable in smb3_fs_context_dup

'rc' in smb3_fs_context_dup is not used and can be removed.

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>