Safer ABI for O_TMPFILE

[suggested by Rasmus Villemoes] make O_DIRECTORY | O_RDWR part of O_TMPFILE;
that will fail on old kernels in a lot more cases than what I came up with.
And make sure O_CREAT doesn't get there...

Change-Id: I4818563d79ca1abf9ea99f5ccea9317eb2f3b678
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>