KVM: struct kvm_memory_slot.flags -> u32

struct kvm_userspace_memory_region.flags is a u32 with a comment that
bits 0 ~ 15 are visible to userspace and the other bits are reserved
for kvm internal use.  KVM_MEMSLOT_INVALID is the only internal use
flag and it has a comment that bits 16 ~ 31 are internally used and
the other bits are visible to userspace.

Therefore, let's define this as a u32 so we don't waste bytes on LP64
systems.  Move to the end of the struct for alignment.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>