x86/mce: use offsetof() to determine section offset

There is no reason to use pointer difference.
A violation of MISRA C Rule 18.2 ("Subtraction between pointers
shall only be applied to pointers that address elements of the
same array") is also resolved because the object to the subtraction
is applied is not an array.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/cpu/mcheck/mce-apei.c

@@ -74,7 +74,7 @@ int apei_write_mce(struct mce *m)
 	rcd.hdr.record_id = cper_next_record_id();
 	rcd.hdr.flags = CPER_HW_ERROR_FLAGS_PREVERR;
 
-	rcd.sec_hdr.section_offset = (void *)&rcd.mce - (void *)&rcd;
+	rcd.sec_hdr.section_offset = offsetof(struct cper_mce_record, mce);
 	rcd.sec_hdr.section_length = sizeof(rcd.mce);
 	rcd.sec_hdr.revision = CPER_SEC_REV;
 	/* fru_id and fru_text is invalid */