The error code that is returned in response to starting a mutually
exclusive operation when there is one already running got silently
changed from EINVAL to EINPROGRESS by 
5ac00add.  Returning EINPROGRESS
to, say, add_dev, when rm_dev is running is misleading.  Furthermore,
the operation itself may want to use EINPROGRESS for other purposes.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
                        1)) {
                pr_info("btrfs: dev add/delete/balance/replace/resize operation in progress\n");
                mnt_drop_write_file(file);
-               return -EINPROGRESS;
+               return -EINVAL;
        }
 
        mutex_lock(&root->fs_info->volume_mutex);
        if (atomic_xchg(&root->fs_info->mutually_exclusive_operation_running,
                        1)) {
                pr_info("btrfs: dev add/delete/balance/replace/resize operation in progress\n");
-               return -EINPROGRESS;
+               return -EINVAL;
        }
        ret = mnt_want_write_file(file);
        if (ret) {
        if (atomic_xchg(&root->fs_info->mutually_exclusive_operation_running,
                        1)) {
                pr_info("btrfs: dev add/delete/balance/replace/resize operation in progress\n");
-               return -EINPROGRESS;
+               return -EINVAL;
        }
 
        mutex_lock(&root->fs_info->volume_mutex);
                        1)) {
                pr_info("btrfs: dev add/delete/balance/replace/resize operation in progress\n");
                mnt_drop_write_file(file);
-               return -EINPROGRESS;
+               return -EINVAL;
        }
 
        mutex_lock(&root->fs_info->volume_mutex);