nfs: get rid of mount_info ->fill_super()

The only possible values are nfs_fill_super and nfs_clone_super.  The
latter is used only when crossing into a submount and it is almost
identical to the former; the only differences are
	* ->s_time_gran unconditionally set to 1 (even for v2 mounts).
Regression dating back to 2012, actually.
	* ->s_blocksize/->s_blocksize_bits set to that of parent.

Rather than messing with the method, stash ->s_blocksize_bits in
mount_info in submount case and after the (now unconditional)
call of nfs_fill_super() override ->s_blocksize/->s_blocksize_bits
if that has been set.

Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>