client: be careful about replacing dentries during readdir assimilation

client: be careful about replacing dentries during readdir assimilation

When we are assimilate readdir results into our cache, we need to be more
careful about replacing existing dentries.  We were calling
insert_dentry_inode(), which would replace a name if it already exists,
which might include pd->first, an active iterator.

Move the dentry link/relink into the caller (where we already have an
iterator pointing to the existing item, if any).  Then update the dentry
lease information separately.

Fixes: #1391
Signed-off-by: Sage Weil <sage@newdream.net>