xen/debug: WARN_ON when identity PFN has no _PAGE_IOMAP flag set.

Only enabled if XEN_DEBUG is enabled. We print a warning
when:

 pfn_to_mfn(pfn) == pfn, but no VM_IO (_PAGE_IOMAP) flag set
	(and pfn is an identity mapped pfn)
 pfn_to_mfn(pfn) != pfn, and VM_IO flag is set.
	(ditto, pfn is an identity mapped pfn)

[v2: Make it dependent on CONFIG_XEN_DEBUG instead of ..DEBUG_FS]
[v3: Fix compiler warning]

Reviewed-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>