x86: fix bug_line()

x86: fix bug_line()

Due to the packing into a bit field together with a relocated field,
the computation can overflow when the relocated field ends up getting a
negative value stored. Hence it isn't sufficient to correct the value
by 1 in this case, but we also need to mask the result to the width of
the original bit field.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>