staging: greybus: loopback_test: fix potential path truncation

Newer GCC warns about a possible truncation of a generated sysfs path
name as we're concatenating a directory path with a file name and
placing the result in a buffer that is half the size of the maximum
length of the directory path (which is user controlled).

loopback_test.c: In function 'open_poll_files':
loopback_test.c:651:31: warning: '%s' directive output may be truncated writing up to 511 bytes into a region of size 255 [-Wformat-truncation=]
  651 |   snprintf(buf, sizeof(buf), "%s%s", dev->sysfs_entry, "iteration_count");
      |                               ^~
loopback_test.c:651:3: note: 'snprintf' output between 16 and 527 bytes into a destination of size 255
  651 |   snprintf(buf, sizeof(buf), "%s%s", dev->sysfs_entry, "iteration_count");
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix this by making sure the buffer is large enough the concatenated
strings.

Fixes: 6b0658f6 ("greybus: tools: Add tools directory to greybus repo and add loopback")
Fixes: 9250c0ee ("greybus: Loopback_test: use poll instead of inotify")
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20200312110151.22028-3-johan@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/greybus/tools/loopback_test.c

@@ -637,7 +637,7 @@ int find_loopback_devices(struct loopback_test *t)
 static int open_poll_files(struct loopback_test *t)
 {
 	struct loopback_device *dev;
-	char buf[MAX_STR_LEN];
+	char buf[MAX_SYSFS_PATH + MAX_STR_LEN];
 	char dummy;
 	int fds_idx = 0;
 	int i;