always reserve elfcore header memory in crash kernel

elfcore header memory needs to be reserved in a crash kernel.  This means
that the relevant code should be protected by CONFIG_CRASH_DUMP rather
than CONFIG_PROC_VMCORE.

Signed-off-by: Simon Horman <horms@verge.net.au>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>