Btrfs: atomically set inode->i_flags in btrfs_update_iflags

This change is based on the corresponding recent change for ext4:

  ext4: atomically set inode->i_flags in ext4_set_inode_flags()

That has the following commit message that applies to btrfs as well:

  "Use cmpxchg() to atomically set i_flags instead of clearing out the
   S_IMMUTABLE, S_APPEND, etc. flags and then setting them from the
   EXT4_IMMUTABLE_FL, EXT4_APPEND_FL flags, since this opens up a race
   where an immutable file has the immutable flag cleared for a brief
   window of time."

Replacing EXT4_IMMUTABLE_FL and EXT4_APPEND_FL with BTRFS_INODE_IMMUTABLE
and BTRFS_INODE_APPEND, respectively.
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Reviewed-by: NSatoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: NFilipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/ioctl.c

@@ -136,19 +136,22 @@ static unsigned int btrfs_flags_to_ioctl(unsigned int flags)
 void btrfs_update_iflags(struct inode *inode)
 {
 	struct btrfs_inode *ip = BTRFS_I(inode);
-
-	inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
+	unsigned int new_fl = 0;
 
 	if (ip->flags & BTRFS_INODE_SYNC)
-		inode->i_flags |= S_SYNC;
+		new_fl |= S_SYNC;
 	if (ip->flags & BTRFS_INODE_IMMUTABLE)
-		inode->i_flags |= S_IMMUTABLE;
+		new_fl |= S_IMMUTABLE;
 	if (ip->flags & BTRFS_INODE_APPEND)
-		inode->i_flags |= S_APPEND;
+		new_fl |= S_APPEND;
 	if (ip->flags & BTRFS_INODE_NOATIME)
-		inode->i_flags |= S_NOATIME;
+		new_fl |= S_NOATIME;
 	if (ip->flags & BTRFS_INODE_DIRSYNC)
-		inode->i_flags |= S_DIRSYNC;
+		new_fl |= S_DIRSYNC;
+
+	set_mask_bits(&inode->i_flags,
+		      S_SYNC | S_APPEND | S_IMMUTABLE | S_NOATIME | S_DIRSYNC,
+		      new_fl);
 }
 
 /*