fuse: fix fallocate vs. ftruncate race

A former patch introducing FUSE_I_SIZE_UNSTABLE flag provided detailed
description of races between ftruncate and anyone who can extend i_size:

> 1. As in the previous scenario fuse_dentry_revalidate() discovered that i_size
> changed (due to our own fuse_do_setattr()) and is going to call
> truncate_pagecache() for some  'new_size' it believes valid right now. But by
> the time that particular truncate_pagecache() is called ...
> 2. fuse_do_setattr() returns (either having called truncate_pagecache() or
> not -- it doesn't matter).
> 3. The file is extended either by write(2) or ftruncate(2) or fallocate(2).
> 4. mmap-ed write makes a page in the extended region dirty.

This patch adds necessary bits to fuse_file_fallocate() to protect from that
race.
Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: stable@vger.kernel.org

fs/fuse/file.c

@@ -2467,6 +2467,7 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 {
 	struct fuse_file *ff = file->private_data;
 	struct inode *inode = file->f_inode;
+	struct fuse_inode *fi = get_fuse_inode(inode);
 	struct fuse_conn *fc = ff->fc;
 	struct fuse_req *req;
 	struct fuse_fallocate_in inarg = {
@@ -2495,6 +2496,9 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 		}
 	}
 
+	if (!(mode & FALLOC_FL_KEEP_SIZE))
+		set_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
+
 	req = fuse_get_req_nopages(fc);
 	if (IS_ERR(req)) {
 		err = PTR_ERR(req);
@@ -2527,6 +2531,9 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
 	fuse_invalidate_attr(inode);
 
 out:
+	if (!(mode & FALLOC_FL_KEEP_SIZE))
+		clear_bit(FUSE_I_SIZE_UNSTABLE, &fi->state);
+
 	if (lock_inode)
 		mutex_unlock(&inode->i_mutex);