mds: allow stray directories to fragment and switch from 10 stray directories to 1
Stray directories can become too full which can result in unexpected ENOSPC errors. See for example, #41778.
Evaluate whether it's feasible to allow fragmentation on strays. It is probably a conservative barrier erected for when fragmentation wasn't considered stable (pre-luminous). The use of 10 stray directories was a workaround. Using only 1 with fragmentation is much simpler.