[IA64] vmcore_find_descriptor_size should be in __init

vmcore_find_descriptor_size() is only called by
reserve_elfcorehdr(), which is in __init, so it seems to me that
vmcore_find_descriptor_size() should be there too.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Tony Luck <tony.luck@intel.com>