NFS: Add nfs_alloc_parsed_mount_data

Allocating nfs_parsed_mount_data and setting up the defaults is nearly
the same for both nfs and nfs4 mounts.

Both paths seem to use nfs_validate_transport_protocol(), so setting a
default value for nfs_server.protocol ought to be unnecessary.

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