NFSv4: Fix an NFSv4 mount regression

The helper nfs_fs_mount() will always call nfs4_try_mount with the
mount_info->fill_super argument pointing to nfs_fill_super, which is
NFSv2/v3 only.
Fix is to have nfs4_try_mount replace it with nfs4_fill_super.

The regression was introduced by commit c40f8d1d (NFS: Create a common
fs_mount() function)

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