Btrfs: serialize flushers in reserve_metadata_bytes

We keep having problems with early enospc, and that's because our method of
making space is inherently racy.  The problem is we can have one guy trying to
make space for himself, and in the meantime people come in and steal his
reservation.  In order to stop this we make a waitqueue and put anybody who
comes into reserve_metadata_bytes on that waitqueue if somebody is trying to
make more space.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>

fs/btrfs/ctree.h

@@ -756,6 +756,8 @@ struct btrfs_space_info {
 				   chunks for this space */
 	unsigned int chunk_alloc:1;	/* set if we are allocating a chunk */
 
+	unsigned int flush:1;		/* set if we are trying to make space */
+
 	unsigned int force_alloc;	/* set if we need to force a chunk
 					   alloc for this space */
 
@@ -766,6 +768,7 @@ struct btrfs_space_info {
 	spinlock_t lock;
 	struct rw_semaphore groups_sem;
 	atomic_t caching_threads;
+	wait_queue_head_t wait;
 };
 
 struct btrfs_block_rsv {