cpumask.h: silence warning with -Wsign-compare

Silence the warning when building with -Wsign-compare when cpumask.h
is included:

include/linux/cpumask.h: In function ‘cpumask_parse’:
include/linux/cpumask.h:603:26: warning: signed and unsigned type in conditional expression [-Wsign-compare]
  int len = nl ? nl - buf : strlen(buf);
                          ^

V2: Rusty pointed out that unsigned should be used instead.

Signed-off-by: Brian W Hart <hartb@linux.vnet.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>