[ARM] 4661/1: fix do_undefinstr wrt the enabling of IRQs

The lock is acquired with spin_lock_irqsave() and released in the
not-found case with spin_unlock_irqrestore().

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>