ksmbd: Remove a redundant zeroing of memory

fill_transform_hdr() has only one caller that already clears tr_buf (it is
kzalloc'ed).

So there is no need to clear it another time here.

Remove the superfluous memset() and add a comment to remind that the caller
must clear the buffer.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Hyunchul Lee <hyc.lee@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>