audit: create private file name copies when auditing inodes

Unfortunately, while commit 4a928436 ("audit: correctly record file
names with different path name types") fixed a problem where we were
not recording filenames, it created a new problem by attempting to use
these file names after they had been freed.  This patch resolves the
issue by creating a copy of the filename which the audit subsystem
frees after it is done with the string.

At some point it would be nice to resolve this issue with refcounts,
or something similar, instead of having to allocate/copy strings, but
that is almost surely beyond the scope of a -rcX patch so we'll defer
that for later.  On the plus side, only audit users should be impacted
by the string copying.

Reported-by: Toralf Foerster <toralf.foerster@gmx.de>
Signed-off-by: Paul Moore <pmoore@redhat.com>