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>