fs/vfs: ftruncate produce EINVAL if fd is not open for writing

As posix spec, ftruncate should produce EBADF or EINVAL error,
if the file descriptor fd is not open for writing.

Change the behavior to be similar to Linux and pass ltp open_posix test:
testcases/open_posix_testsuite/conformance/interfaces/shm_open/20-1.c

https://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html
https://man7.org/linux/man-pages/man2/ftruncate.2.html

Signed-off-by: fangxinyong <fangxinyong@xiaomi.com>

fs/vfs/fs_truncate.c

@@ -57,7 +57,7 @@ int file_truncate(FAR struct file *filep, off_t length)
   if ((filep->f_oflags & O_WROK) == 0)
     {
       fwarn("WARNING: Cannot truncate a file opened read-only\n");
-      return -EBADF;
+      return -EINVAL;
     }
 
   /* Is this inode a registered mountpoint? Does it support the