x86: fix off-by-one in find_next_zero_string

Fix an off-by-one error in find_next_zero_string which prevents
allocating the last bit.

[ tglx: arch/x86 adaptation ]

Signed-off-by: Andrew Hastings <abh@cray.com> on behalf of Cray Inc.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>