x86/shadow: Use ERR_PTR infrastructure for sh_emulate_map_dest()

x86/shadow: Use ERR_PTR infrastructure for sh_emulate_map_dest()

sh_emulate_map_dest() predates the introduction of the generic ERR_PTR()
infrastructure, but take the opportunity to avoid opencoding it.

The chosen error constants require need to be negative to work with IS_ERR(),
but no other changes.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>