passthrough_ll: set correct keep_cache flag in opendir (#1083)

passthrough_ll: set correct keep_cache flag in opendir (#1083)

In cache=always mode, set keep_cache flag in opendir(), same as done
in open() and same as passthrough_hp does in opendir().

In the default cache=auto mode, use readdir cache, but do not set
keep_cache, same as regular files use page cache for an open file,
but do not keep_cache for a new open.

Note that passthrough_hp by default behaves the same as passthrough_ll
cache=always mode and supports the cache=never mode with --nocache, but
it does not support the equivalent of cache=auto mode.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>