vmcore, sysfs: export ELF note segment size instead of vmcoreinfo data size

vmcore, sysfs: export ELF note segment size instead of vmcoreinfo data size

Currently, vmcoreinfo exports data part only, but kexec-tool sets it in
p_memsz member as a whole ELF note segment size.  Due to this, it would be
no problem on the current ELF note segment size, but if it grows in the
future, then read possibly doesn't reach ELF note header in larger p_memsz
position, failing to read a whole ELF segment.

Note: kexec-tools assigns PAGE_SIZE to p_memsz for other ELF note types.
Due to the above reason, the same issue occurs if actual ELF note data
exceeds (PAGE_SIZE - 2 * KEXEC_NOTE_HEAD_BYTES).

Signed-off-by: HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>
Cc: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>