crashdump/x86: increase CRASH_MAX_MEMORY_RANGES to 32k

virtio-mem in Linux adds/removes individual memory blocks (e.g., 128 MB
each). Linux merges adjacent memory blocks added by virtio-mem devices, but
we can still end up with a very sparse memory layout when unplugging
memory in corner cases.

Let's increase the maximum number of crash memory ranges from ~2k to 32k.
32k should be sufficient for a very long time.

e_phnum field in the header is 16 bits wide, so we can fit a maximum of
~64k entries in there, shared with other entries (i.e., CPU). Therefore,
using up to 32k memory ranges is fine. (if we ever need more than ~64k,
we can switch to the sh_info field)

Move the temporary xen ranges off the stack, dynamically allocating
memory for them.

Note: We don't have to increase MAX_MEMORY_RANGES, because virtio-mem
added memory is driver managed and always detected and added by a
driver in the kexec'ed kernel; for ordinary kexec, we must not expose
these ranges in the firmware-provided memmap.

Cc: Simon Horman <horms@verge.net.au>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>