Btrfs: remove last_log_alloc allocator optimization

The tree logging code was trying to separate tree log allocations
from normal metadata allocations to improve writeback patterns during
an fsync.

But, the code was not effective and ended up just mixing tree log
blocks with regular metadata.  That seems to be working fairly well,
so the last_log_alloc code can be removed.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -672,7 +672,6 @@ struct btrfs_fs_info {
 	u64 delalloc_bytes;
 	u64 last_alloc;
 	u64 last_data_alloc;
-	u64 last_log_alloc;
 
 	spinlock_t ref_cache_lock;
 	u64 total_ref_cache_size;

fs/btrfs/extent-tree.c

@@ -2228,13 +2228,6 @@ static int noinline find_free_extent(struct btrfs_trans_handle *trans,
 	if ((data & BTRFS_BLOCK_GROUP_DATA) && btrfs_test_opt(root, SSD))
 		last_ptr = &root->fs_info->last_data_alloc;
 
-	if (root->root_key.objectid == BTRFS_TREE_LOG_OBJECTID) {
-		last_ptr = &root->fs_info->last_log_alloc;
-		if (!last_ptr == 0 && root->fs_info->last_alloc) {
-			*last_ptr = root->fs_info->last_alloc + empty_cluster;
-		}
-	}
-
 	if (last_ptr) {
 		if (*last_ptr)
 			hint_byte = *last_ptr;

fs/btrfs/transaction.c

@@ -61,7 +61,6 @@ static noinline int join_transaction(struct btrfs_root *root)
 		root->fs_info->generation++;
 		root->fs_info->last_alloc = 0;
 		root->fs_info->last_data_alloc = 0;
-		root->fs_info->last_log_alloc = 0;
 		cur_trans->num_writers = 1;
 		cur_trans->num_joined = 0;
 		cur_trans->transid = root->fs_info->generation;