Flexible client buffer growth

Flexible client buffer growth

Allocates the 2k-directory-buffers for the client as needed, in
small increments, rather than a fixed-length area upon startup.

Change-Id: I6061b71d40284a0059e54377e3b374fee9b86816
Reviewed-on: http://gerrit.openafs.org/799
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>