Handle EINVAL error return of BTRFS_IOC_SYNC
Handle EINVAL error return of BTRFS_IOC_SYNC

(cherry picked from commit dced36d9aa1f2bc540e06a5fe6d67d3140c2e504)

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend