ext4: properly check for dirty state in ext4_inode_datasync_dirty()

ext4_inode_datasync_dirty() needs to return 'true' if the inode is
dirty, 'false' otherwise, but the logic seems to be incorrectly changed
by commit aa75f4d3da ("ext4: main fast-commit commit path").

This introduces a problem with swap files that are always failing to be
activated, showing this error in dmesg:

 [   34.406479] swapon: file is not committed

Simple test case to reproduce the problem:

  # fallocate -l 8G swapfile
  # chmod 0600 swapfile
  # mkswap swapfile
  # swapon swapfile

Fix the logic to return the proper state of the inode.

Link: https://lore.kernel.org/lkml/20201024131333.GA32124@xps-13-7390
Fixes: 8016e29f43 ("ext4: fast commit recovery path")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201027044915.2553163-1-harshadshirwadkar@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -3308,9 +3308,11 @@ static bool ext4_inode_datasync_dirty(struct inode *inode)
 	if (journal) {
 		if (jbd2_transaction_committed(journal,
 			EXT4_I(inode)->i_datasync_tid))
-			return true;
-		return atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid) >=
+			return false;
+		if (test_opt2(inode->i_sb, JOURNAL_FAST_COMMIT))
+			return atomic_read(&EXT4_SB(inode->i_sb)->s_fc_subtid) <
 				EXT4_I(inode)->i_fc_committed_subtid;
+		return true;
 	}
 
 	/* Any metadata buffers to write? */