vfs: deny fallocate() on directory

There was an obscure use case of fallocate of directory inode
in the vfs helper with the comment:
"Let individual file system decide if it supports preallocation
 for directories or not."

But there is no in-tree file system that implements fallocate
for directory operations.

Deny an attempt to fallocate a directory with EISDIR error.

This change is needed prior to converting sb_start_write()
to  file_start_write(), so freeze protection is correctly
handled for cases of fallocate file and blockdev.

Cc: linux-api@vger.kernel.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/open.c

@@ -301,12 +301,10 @@ int vfs_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
 	if (S_ISFIFO(inode->i_mode))
 		return -ESPIPE;
 
-	/*
-	 * Let individual file system decide if it supports preallocation
-	 * for directories or not.
-	 */
-	if (!S_ISREG(inode->i_mode) && !S_ISDIR(inode->i_mode) &&
-	    !S_ISBLK(inode->i_mode))
+	if (S_ISDIR(inode->i_mode))
+		return -EISDIR;
+
+	if (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))
 		return -ENODEV;
 
 	/* Check for wrap through zero too */