CIFS: Clarify SMB1 code for delete

CIFS: Clarify SMB1 code for delete

Coverity also complains about the way we calculate the offset
(starting from the address of a 4 byte array within the
header structure rather than from the beginning of the struct
plus 4 bytes) for SMB1 SetFileDisposition (which is used to
unlink a file by setting the delete on close flag).  This
changeset doesn't change the address but makes it slightly
clearer.

Addresses-Coverity: 711524 ("Out of bounds write")
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>