_scheduleDatabaseRefresh already holds the _mutex when making this call:
https://github.com/mongodb/mongo/blob/2393ed14199eddebe25e56bc0a8fe1d47f2297cf/src/mongo/s/catalog_cache.cpp#L535
Similarly inside _scheduleCollectionRefresh:
https://github.com/mongodb/mongo/blob/2393ed14199eddebe25e56bc0a8fe1d47f2297cf/src/mongo/s/catalog_cache.cpp#L621