security: Update file_set_fowner documentation

Highlight that the file_set_fowner hook is now called with a lock held.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: James Morris <jmorris@namei.org>
Cc: Jann Horn <jannh@google.com>
Cc: Ondrej Mosnacek <omosnace@redhat.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge E. Hallyn <serge@hallyn.com>
Cc: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/security.c

@@ -3048,6 +3048,8 @@ int security_file_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
  * Save owner security information (typically from current->security) in
  * file->f_security for later use by the send_sigiotask hook.
  *
+ * This hook is called with file->f_owner.lock held.
+ *
  * Return: Returns 0 on success.
  */
 void security_file_set_fowner(struct file *file)