With O_TMPFILE, open(2) takes a mode argument.

Strictly, the mode isn't really meaningful unless you supply O_EXCL,
but the kernel will take it and fstat will return it even if you
never give the file a name.

Also warn for O_TMPFILE without a mode at compile time where possible.

Bug: N/A
Test: ran tests
Change-Id: I729b6d6e6190676fd017a1190b6200bf9abdbfd8