CIFS: Clarify SMB1 code for POSIX Lock

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 PosixLock. This changeset
doesn't change the address but makes it slightly clearer.

Addresses-Coverity: 711520 ("Out of bounds write")
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>