rgw_file: rgw_readdir can't list multi-segment dirs

rgw_file: rgw_readdir can't list multi-segment dirs

This issue has one root cause in librgw, namely that the marker
argument to these requests was incorrectly formatted (though the
marker cache was working as intended).

Secondarily, for nfs-ganesha users, there is a compounding issue
that the RGW fsal was required by "temporary" convention to
populate the entire dirent cache for a directory on a single
readdir() invocation--the cache_inode/mdcache implementations
invariantly pass (before future 2.5 changesets, currently in
progress) a null pointer for the start cookie offset, intended
to convey this.

Fixes: http://tracker.ceph.com/issues/18991
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>