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>

include/linux/cpumask.h

@@ -600,7 +600,7 @@ static inline int cpulist_scnprintf(char *buf, int len,
 static inline int cpumask_parse(const char *buf, struct cpumask *dstp)
 {
 	char *nl = strchr(buf, '\n');
-	int len = nl ? nl - buf : strlen(buf);
+	unsigned int len = nl ? (unsigned int)(nl - buf) : strlen(buf);
 
 	return bitmap_parse(buf, len, cpumask_bits(dstp), nr_cpumask_bits);
 }