cifsd: fix memleak in ksmbd_vfs_stream_write()

Before assigning wbuf to stream_buf, memory allocate in
ksmbd_vfs_getcasexattr() need be freed.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>