client: only link directories into hierarchy once
client: only link directories into hierarchy once

Directories can only link into the hierarchy once.  We assert as much
in readdir_r_cb().  Fix link() so that it unlinked the directory from the
old location when relinking somewhere new.  Be careful to do this after
we take inode refs to avoid any unpleasantness.

Fixes: #1429
Reported-by: Sam Lang <08091de2673a2a98b30d2d3346744b3287b21c24@gmail.com>
Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph