NFS: Clean up NFSROOT command line parsing

Clean up: To reduce confusion, rename nfs_root_name as nfs_root_parms,
as this buffer contains more than just the name of the remote server.

Introduce documenting comments around nfs_root_setup().

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>