vmcoreinfo: use the existing offsetof() for VMCOREINFO_OFFSET()

It is better that the existing offsetof() is used for VMCOREINFO_OFFSET().

This discussion is the following:
http://www.ussg.iu.edu/hypermail/linux/kernel/0709.3/0584.html

Signed-off-by: Ken'ichi Ohmichi <oomichi@mxs.nes.nec.co.jp>
Acked-by: Simon Horman <horms@verge.net.au>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kexec.h

@@ -137,7 +137,7 @@ unsigned long paddr_vmcoreinfo_note(void);
 			      (unsigned long)sizeof(struct name))
 #define VMCOREINFO_OFFSET(name, field) \
 	vmcoreinfo_append_str("OFFSET(%s.%s)=%lu\n", #name, #field, \
-			      (unsigned long)&(((struct name *)0)->field))
+			      (unsigned long)offsetof(struct name, field))
 #define VMCOREINFO_LENGTH(name, value) \
 	vmcoreinfo_append_str("LENGTH(%s)=%lu\n", #name, (unsigned long)value)
 #define VMCOREINFO_NUMBER(name) \