ARM: fix address limit restoration for undefined instructions

During boot, sometimes the kernel will test to see if an instruction
causes an undefined instruction exception.  Unfortunately, the exit
path for these exceptions did not restore the address limit, which
causes the rootfs mount code to fail.  Fix the missing address limit
restoration.

Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>