compat/memory: avoid UB shifts in XENMEM_exchange handling

compat/memory: avoid UB shifts in XENMEM_exchange handling

Add an early basic check, yielding the same error code as the more
thorough one the main handler would produce.

Fixes: b8a7efe8528a ("Enable compatibility mode operation for HYPERVISOR_memory_op")
Reported-by: Manuel Andreas <manuel.andreas@tum.de>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Jason Andryuk <jason.andryuk@amd.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>