Btrfs: remove unused mutex from struct 'btrfs_fs_info'

The code using 'ordered_extent_flush_mutex' mutex has removed by below
commit.
 - 8d875f95da43c6a8f18f77869f2ef26e9594fecc
   btrfs: disable strict file flushes for renames and truncates
But the mutex still lives in struct 'btrfs_fs_info'.

So, this patch removes the mutex from struct 'btrfs_fs_info' and its
initialization code.

Signed-off-by: Byongho Lee <bhlee.kernel@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ctree.h

@@ -1518,12 +1518,6 @@ struct btrfs_fs_info {
 	 */
 	struct mutex ordered_operations_mutex;
 
-	/*
-	 * Same as ordered_operations_mutex except this is for ordered extents
-	 * and not the operations.
-	 */
-	struct mutex ordered_extent_flush_mutex;
-
 	struct rw_semaphore commit_root_sem;
 
 	struct rw_semaphore cleanup_work_sem;

fs/btrfs/disk-io.c

@@ -2608,7 +2608,6 @@ int open_ctree(struct super_block *sb,
 
 
 	mutex_init(&fs_info->ordered_operations_mutex);
-	mutex_init(&fs_info->ordered_extent_flush_mutex);
 	mutex_init(&fs_info->tree_log_mutex);
 	mutex_init(&fs_info->chunk_mutex);
 	mutex_init(&fs_info->transaction_kthread_mutex);