Btrfs: optimize fsync for the single writer case

This patch optimizes the tree logging stuff so it doesn't always wait 1 jiffie
for new people to join the logging transaction if there is only ever 1 writer.
This helps a little bit with latency where we have something like RPM where it
will fdatasync every file it writes, and so waiting the 1 jiffie for every
fdatasync really starts to add up.

Signed-off-by: Josef Bacik <jbacik@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -1010,6 +1010,8 @@ struct btrfs_root {
 	atomic_t log_commit[2];
 	unsigned long log_transid;
 	unsigned long log_batch;
+	pid_t log_start_pid;
+	bool log_multiple_pids;
 
 	u64 objectid;
 	u64 last_trans;