[ARM] call undefined instruction exception handler with irqs enabled

Aaro says:
> With spinlock debugs enabled I get might_sleep() warnings when using
> ptrace.

tracked down to a missing enable_irq before calling do_undefinstr().
Reported-by: NAaro Koskinen <aaro.koskinen@nokia.com>
Tested-by: NAaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/kernel/entry-armv.S

@@ -650,6 +650,7 @@ ENTRY(fp_enter)
 no_fp:	mov	pc, lr
 
 __und_usr_unknown:
+	enable_irq
 	mov	r0, sp
 	adr	lr, ret_from_exception
 	b	do_undefinstr