avr32: Fix oops on unaligned user access

The unaligned address exception handler (and others) does not scan the
fixup tables before oopsing. This is bad because it means passing a
badly aligned pointer from user space might crash the kernel.

Fix this by scanning the fixup tables in _exception(). This should
resolve the issue for unaligned addresses as well as other less common
exceptions that might be happening during a userspace access. The page
fault handler already does fixup processing.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>