roundrobin_datadirs was returning any .db file at any depth in the
accounts/containers structure. Since xfs corruption can cause such
files to appear in odd places at times (only happened on one drive of
ours so far, but still...), I've refactored this function to only
return .db files at the proper depth.
Change-Id: Id06ef6584941f8a572e286f69dfa3d96fe451355
4e5889d6ce