client: make less noise when unlinking during readdir

Skip, but do not talk about, NULL dentries.

Signed-off-by: Sage Weil <sage@inktank.com>
client: use __func__ instead of incorrect function name in insert_readdir_results

Signed-off-by: Sage Weil <sage@inktank.com>
client: fix whitespace in stat relpath

Signed-off-by: Sage Weil <sage@inktank.com>