altp2m: don't attempt to unshare pages during change_altp2m_gfn op

altp2m: don't attempt to unshare pages during change_altp2m_gfn op

Attempting to change gfn mappings with altp2m on a memory shared page results
in a lock-order violation (mm locking order violation: 282 > 254), which
crashes the hypervisor. Don't attempt to automatically unshare such pages and
just fall back to failing the op if the page type is not correct.

Signed-off-by: Tamas K Lengyel <tamas.lengyel@zentific.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>