xen/balloon: Fix crash when ballooning on x86 32 bit PAE

xen/balloon: Fix crash when ballooning on x86 32 bit PAE

Commit 55b3da98a40dbb3776f7454daf0d95dde25c33d2 (xen/balloon: find
non-conflicting regions to place hotplugged memory) caused a
regression in 4.4.

When ballooning on an x86 32 bit PAE system with close to 64 GiB of
memory, the address returned by allocate_resource may be above 64 GiB.
When using CONFIG_SPARSEMEM, this setup is limited to using physical
addresses < 64 GiB.  When adding memory at this address, it runs off
the end of the mem_section array and causes a crash.  Instead, fail
the ballooning request.

Signed-off-by: Ross Lagerwall <[email protected]>
Cc: <[email protected]> # 4.4+
Signed-off-by: David Vrabel <[email protected]>