rgw_file: restore (corrected) fix for dir "partial match" (return of FLAG_EXACT_MATCH)
Allow callers of rgw_lookup() on objects attested in an rgw_readdir() callback the ability to bypass exact match in RGWLibFS::stat_leaf() case 2, but restore exact match enforcement for general lookups.
This preserves required common_prefix namespace behavior, but prevents clients from eerily permitting things like "cd sara0" via partial name match on "sara01." With the original common_prefix name fix and namespace invalidates, this behavior was "somewhat harmless," in that creating objects in the non-existent directory had the effect of creating it. Still, nobody expects "cd" to create directories, do they?