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>

Author: smfrench

fs/cifs/cifssmb.c

@@ -2537,8 +2537,9 @@ CIFSSMBPosixLock(const unsigned int xid, struct cifs_tcon *tcon,
 	pSMB->TotalDataCount = pSMB->DataCount;
 	pSMB->TotalParameterCount = pSMB->ParameterCount;
 	pSMB->ParameterOffset = cpu_to_le16(param_offset);
+	/* SMB offsets are from the beginning of SMB which is 4 bytes in, after RFC1001 field */
 	parm_data = (struct cifs_posix_lock *)
-			(((char *) &pSMB->hdr.Protocol) + offset);
+			(((char *)pSMB) + offset + 4);
 
 	parm_data->lock_type = cpu_to_le16(lock_type);
 	if (waitFlag) {