FileStore: FDCache hit is not creating Index objects

If caller is requesting to populate Index objects, it is not honoring
that in case of FDCache hit. The logic is changed to populate it
if there is a cache hit and the param Index object is not null.

Signed-off-by: Somnath Roy <somnath.roy@sandisk.com>
Merge branch 'wip-sd-filestore-optimization' into wip-sd-performance-testing