x86: fix sys_sigreturn() return type to be long, not unsigned long

Same as with other system calls, sys_sigreturn() should return a value
of type long, not unsigned long. This also matches the behaviour for
IA32_EMULATION, see sys32_sigreturn() in arch/x86/ia32/ia32_signal.c .

Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: x86@kernel.org
Cc: Michael Tautschnig <tautschn@amazon.co.uk>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

arch/x86/include/asm/syscalls.h

@@ -35,7 +35,7 @@ asmlinkage long sys_get_thread_area(struct user_desc __user *);
 #ifdef CONFIG_X86_32
 
 /* kernel/signal.c */
-asmlinkage unsigned long sys_sigreturn(void);
+asmlinkage long sys_sigreturn(void);
 
 /* kernel/vm86_32.c */
 struct vm86_struct;

arch/x86/kernel/signal.c

@@ -601,7 +601,7 @@ static int x32_setup_rt_frame(struct ksignal *ksig,
  * Do a signal return; undo the signal stack.
  */
 #ifdef CONFIG_X86_32
-asmlinkage unsigned long sys_sigreturn(void)
+asmlinkage long sys_sigreturn(void)
 {
 	struct pt_regs *regs = current_pt_regs();
 	struct sigframe __user *frame;