lib, net: make isodigit() public and use it

There are at least two users of isodigit().  Let's make it a public
function of ctype.h.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/ctype.h b/include/linux/ctype.h
index 8acfe31..653589e 100644
--- a/include/linux/ctype.h
+++ b/include/linux/ctype.h
@@ -61,4 +61,10 @@
 	return c | 0x20;
 }
 
+/* Fast check for octal digit */
+static inline int isodigit(const char c)
+{
+	return c >= '0' && c <= '7';
+}
+
 #endif