util/disk_cache: pass predicate functions file stats directly (v4)

util/disk_cache: pass predicate functions file stats directly (v4)

Since switching to LRU eviction the only user of these predicate
functions now resolves directory entry stats itself so pass them
directly saving calling fstat and strlen twice (and the
expensive strlen is skipped entirely if access time is newer).

v2: Update for empty cache dir detection changes
v3: Fix passing string length to predicate with the +1 for NULL
    termination and also pass sb as pointer
v4: Missed ampersand for passing sb as pointer

Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Timothy Arceri <tarceri@itsqueeze.com>