NFSD: Refactor common code out of dirlist helpers

The dust has settled a bit and it's become obvious what code is
totally common between nfsd_init_dirlist_pages() and
nfsd3_init_dirlist_pages(). Move that common code to SUNRPC.

The new helper brackets the existing xdr_init_decode_pages() API.

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