NFSv4.1: Use more sensible names for 'initialize_mountpoint'

The initialize_mountpoint/uninitialise_mountpoint functions are really about
setting or clearing the layout driver to be used on this filesystem. Change
the names to the more descriptive 'set_layoutdriver/clear_layoutdriver'.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>