cxl/region: Fix region HPA ordering validation

In the Linux kernel, the following vulnerability has been resolved:

cxl/region: Fix region HPA ordering validation

Some regions may not have any address space allocated. Skip them when

validating HPA order otherwise a crash like the following may result:

devm_cxl_add_region: cxl_acpi cxl_acpi.0: decoder3.4: created region9

BUG: kernel NULL pointer dereference, address: 0000000000000000

[..]

RIP: 0010:store_targetN+0x655/0x1740 [cxl_core]

[..]

Call Trace:

<TASK>

kernfs_fop_write_iter+0x144/0x200

vfs_write+0x24a/0x4d0

ksys_write+0x69/0xf0

do_syscall_64+0x3a/0x90

store_targetN+0x655/0x1740:

alloc_region_ref at drivers/cxl/core/region.c:676

(inlined by) cxl_port_attach_region at drivers/cxl/core/region.c:850

(inlined by) cxl_region_attach at drivers/cxl/core/region.c:1290

(inlined by) attach_target at drivers/cxl/core/region.c:1410

(inlined by) store_targetN at drivers/cxl/core/region.c:1453