userfaultfd: selftest: only warn if __NR_userfaultfd is undefined

userfaultfd: selftest: only warn if __NR_userfaultfd is undefined

If __NR_userfaultfd is not yet defined by the arch, warn but still build
and run the userfaultfd selftest successfully.

Signed-off-by: Michael Ellerman <[email protected]>
Signed-off-by: Andrea Arcangeli <[email protected]>
Cc: Dr. David Alan Gilbert <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Thierry Reding <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>