x86/vmap: handle superpages in vmap_to_mfn()

x86/vmap: handle superpages in vmap_to_mfn()

There is simply no guarantee that vmap won't return superpages to the
caller. It can happen if the list of MFNs are contiguous, or we simply
have a large granularity. Although rare, if such things do happen, we
will simply hit BUG_ON() and crash.

Introduce xen_map_to_mfn() to translate any mapped Xen address to mfn
regardless of page size, and wrap vmap_to_mfn() around it.

Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>