um/ptrace: Fix the syscall_trace_leave call

Keep the same semantic as before the commit 26703c63: deallocate
audit context and fake a proper syscall exit.

This fix a kernel panic triggered by the seccomp_bpf test:
> [ RUN      ] global.ERRNO_valid
> BUG: failure at kernel/auditsc.c:1504/__audit_syscall_entry()!
> Kernel panic - not syncing: BUG!

Fixes: 26703c63 ("um/ptrace: run seccomp after ptrace")
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: James Morris <jmorris@namei.org>
Cc: user-mode-linux-devel@lists.sourceforge.net
Signed-off-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

arch/um/kernel/skas/syscall.c

@@ -21,11 +21,11 @@ void handle_syscall(struct uml_pt_regs *r)
 	PT_REGS_SET_SYSCALL_RETURN(regs, -ENOSYS);
 
 	if (syscall_trace_enter(regs))
-		return;
+		goto out;
 
 	/* Do the seccomp check after ptrace; failures should be fast. */
 	if (secure_computing(NULL) == -1)
-		return;
+		goto out;
 
 	/* Update the syscall number after orig_ax has potentially been updated
 	 * with ptrace.
@@ -37,5 +37,6 @@ void handle_syscall(struct uml_pt_regs *r)
 		PT_REGS_SET_SYSCALL_RETURN(regs,
 				EXECUTE_SYSCALL(syscall, regs));
 
+out:
 	syscall_trace_leave(regs);
 }