compat_ioctl: fix FIONREAD on devices

My final cleanup patch for sys_compat_ioctl() introduced a regression on
the FIONREAD ioctl command, which is used for both regular and special
files, but only works on regular files after my patch, as I had missed
the warning that Al Viro put into a comment right above it.

Change it back so it can work on any file again by moving the implementation
to do_vfs_ioctl() instead.

Fixes: 77b90401 ("compat_ioctl: simplify the implementation")
Reported-and-tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Reported-and-tested-by: youling257 <youling257@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

fs/ioctl.c

@@ -523,13 +523,9 @@ static int compat_ioctl_preallocate(struct file *file, int mode,
 
 static int file_ioctl(struct file *filp, unsigned int cmd, int __user *p)
 {
-	struct inode *inode = file_inode(filp);
-
 	switch (cmd) {
 	case FIBMAP:
 		return ioctl_fibmap(filp, p);
-	case FIONREAD:
-		return put_user(i_size_read(inode) - filp->f_pos, p);
 	case FS_IOC_RESVSP:
 	case FS_IOC_RESVSP64:
 		return ioctl_preallocate(filp, 0, p);
@@ -721,6 +717,13 @@ static int do_vfs_ioctl(struct file *filp, unsigned int fd,
 	case FIDEDUPERANGE:
 		return ioctl_file_dedupe_range(filp, argp);
 
+	case FIONREAD:
+		if (!S_ISREG(inode->i_mode))
+			return vfs_ioctl(filp, cmd, arg);
+
+		return put_user(i_size_read(inode) - filp->f_pos,
+				(int __user *)argp);
+
 	default:
 		if (S_ISREG(inode->i_mode))
 			return file_ioctl(filp, cmd, argp);