fs: Fix file mode for O_TMPFILE

O_TMPFILE, like O_CREAT, should respect the requested mode and should
create regular files.

This fixes two bugs: O_TMPFILE required privilege (because the mode
ended up as 000) and it produced bogus inodes with no type.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>