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>