userfaultfd: enhance decoding of struct uffdio_api.features

userfaultfd: enhance decoding of struct uffdio_api.features

As struct uffdio_api.features has read-write semantics,
print the value returned by the kernel only when it differs
from the value passed to the kernel.

* userfaultfd.c (uffdio_ioctl) <UFFDIO_API>: On entering syscall,
save the value of struct uffdio_api.features.  On exiting syscall, do
not print struct uffdio_api.features when it's the same as on entering.
* tests/ioctl_uffdio.c (main): Update expected output.