kclient: fix I_COMPLETE

kclient: fix I_COMPLETE

The previous use of I_READDIR vs I_COMPLETE was flawed, mainly because
the state was maintained on a per-inode basis, but readdir proceeds on a
per-file basis.

Instead of flags, maintain a counter in the inode that is incremented each
time a dentry is released.  When readdir starts, note the counter, and if
it is the same when readdir completes, AND we did not do any forward
seeks on the file handle, AND prepopulate succeeded on each hunk, then we
can set I_COMPLETE.