posixdriver: fix cache fill of versioned buckets

posixdriver: fix cache fill of versioned buckets

This change completes the original intent (hypothesized) to
conditionally set the FLAG_CURRENT bit on just the current
entries during bucket listing cache fill.

This avoids interning 2 copies of the current version of each
object in the listing cache, and also correctly sets the
FLAG_CURRENT bit as required--so the current versions are correctly
reported in versioned listings.

Janky logic to find the current version by explicitly chasing
the symlink target and saving it outside the enumeration scope
has been replaced with proper call to stat() provided by Dang.

Symlink::fill_cache() is no longer used, so removed.

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>