[IA64] Reserve elfcorehdr memory in CONFIG_CRASH_DUMP

IA64 kdump kernel failed to initialize /proc/vmcore in 2.6.28-rc2.
A bug was introduced in this patch commit:

  d9a9855d
  always reserve elfcore header memory in crash kernel

The problem was that the call to reserve_elfcorehdr() should be placed
in CONFIG_CRASH_DUMP rather than in CONFIG_CRASH_KERNEL, which does
not exist.

Signed-off-by: Jay Lan <jlan@sgi.com>
Acked-by: Simon Hormon <horms@verge.net.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>