CIFS: Clarify SMB1 code for UnixSetPathInfo

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 doing SetPathInfo (setattr) when using the Unix
extensions.  This doesn't change the address but makes it
slightly clearer.

Addresses-Coverity: 711528 ("Out of bounds read")
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>