Remove the confusing use of BUFSIZE

Reading BUFSIZ-1 chars (8091) into a BUFSIZE'd buffer (256) is dangerous.

Now only the macro BUFSIZ is used.

Thanks to @prspkt and @sauyon for pointing this out earlier.

src/linux.c

@@ -42,7 +42,7 @@ int checkinterface(void)
 
 int get_stat(void)
 {
-    char buffer[BUFSIZE];
+    char buffer[BUFSIZ];
     char *ptr;
     char *devname;
     unsigned long dump;

src/linux.h

@@ -1,5 +1,2 @@
 int checkinterface(void);
 int get_stat(void);
-#ifdef __linux__
-#define BUFSIZE 256
-#endif