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During O_CREAT we unconditionally audit the parent inode. This makes it
difficult to support a fastpath for O_CREAT when the file already exists
because we have to drop out of RCU lookup needlessly.

We worked around this by checking whether audit was actually active but
that's also suboptimal. Instead, move the audit of the parent inode down
into lookup_open() at a point where it's mostly certain that the file
needs to be created.

This also reduced the inconsistency that currently exists: while audit
on the parent is done independent of whether or no the file already
existed an audit on the file is only performed if it has been created.

By moving the audit down a bit we emit the audit a little later but it
will allow us to simplify the fastpath for O_CREAT significantly.
Signed-off-by: Christian Brauner <brauner@kernel.org>