apparmor: fix uninitialized variable warning in aa_inherit_files

Fix the following warning

All warnings (new ones prefixed by >>):

>> security/apparmor/file.c:737:53: warning: variable 'file' is uninitialized when used here [-Wuninitialized]
     737 |                 int error = aa_file_perm(OP_INHERIT, cred, label, file,
         |                                                                   ^~~~
   security/apparmor/file.c:724:36: note: initialize the variable 'file' to silence this warning
     724 |         struct file *devnull = NULL, *file;
         |                                           ^
         |                                            = NULL
   1 warning generated.

vim +/file +737 security/apparmor/file.c

   719
   720	/* based on selinux's flush_unauthorized_files */
   721	void aa_inherit_files(const struct cred *cred, struct files_struct *files)
   722	{
   723		struct aa_label *label = aa_get_newest_cred_label(cred);
   724		struct file *devnull = NULL, *file;
   725		bool devnull_tried = false;
   726		unsigned int n = 0;
   727
   728		revalidate_tty(cred, label);
   729
   730		/*
   731		 * Revalidate access to inherited open files. Each file is
   732		 * revalidated holding only a reference to it, with no locks
   733		 * held, as revalidation may sleep (e.g. unix socket mediation
   734		 * dropping path refs, or allocating path buffers).
   735		 */
   736		while ((n = iterate_fd(files, n, get_next_file, &file)) != 0) {
 > 737			int error = aa_file_perm(OP_INHERIT, cred, label, file,

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202610292346.ehqNujiE-lkp@intel.com/
Fixes: a913281f60015 ("apparmor: don't revalidate inherited files under files->file_lock")
Signed-off-by: John Johansen <john.johansen@canonical.com>