tools: flock: add NFSv4 compatibility

This patch fixes the LEDE build on mounted NFSv4 shares.

The lock file cannot be opened in read-write mode by default, because
then we cannot use flock(1) to lock executable files.

The read-write mode for lock files is necessary on NFSv4 where
flock(2) is emulated by by fcntl() -- this situation is possible to
detect by flock(2) EBADF error.

The patch consist of the following util-linux/flock commits

http://git.kernel.org/cgit/utils/util-linux/util-linux.git/commit/?id=eb742a1f66d5e3a7c5b43efce741c113f51bef3b

http://git.kernel.org/cgit/utils/util-linux/util-linux.git/commit/?id=caf1ba11a367ad702fb774653daf9ebdcca49d7b

without including the pre kernel 3.4 support.

Signed-off-by: Mathias Kresin <dev@kresin.me>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [minor cleanup]

tools/flock/src/flock.c

@@ -133,6 +133,7 @@ int main(int argc, char *argv[])
   int do_close = 0;
   int err;
   int status;
+  int open_flags = 0;
   char *eon;
   char **cmd_argv = NULL, *sh_c_argv[4];
   const char *filename = NULL;
@@ -266,6 +267,21 @@ int main(int argc, char *argv[])
       if ( timeout_expired )
 	exit(1);		/* -w option set and failed to lock */
       continue;			/* otherwise try again */
+    case EBADF:			/* since Linux 3.4 (commit 55725513) */
+      /* Probably NFSv4 where flock() is emulated by fcntl().
+       * Let's try to reopen in read-write mode.
+       */
+      if (!(open_flags & O_RDWR) &&
+          type != LOCK_SH &&
+          filename &&
+          access(filename, R_OK | W_OK) == 0) {
+
+              close(fd);
+              open_flags = O_RDWR;
+              fd = open(filename, open_flags);
+              break;
+      }
+      /* go through */
     default:			/* Other errors */
       if ( filename )
 	fprintf(stderr, "%s: %s: %s\n", program, filename, strerror(err));