O_TMPFILE detection

[Christoph Hellwig]

I've started implementing support for O_TMPFILE in xfs_io in preparation
for proper testcases and found something rather annoying:

Given that we implicitly pass O_DIRECTORY as part of the full O_TMPFILE
flag an open using O_TMPFILE will simply succeed on a kernel old enough
to not have O_TMPFILE support, and give you a file descriptor for the
directory passed in.  Of course this only works if you open the fd
read-only which isn't all that useful for real temporary files, but it
would be the natural choice for a feature test.

I guess we can't really fix this up any more, but does anyone have a
good idea how to document this properly in the man page?
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