NFS: Fix NFS KConfig typos

NFS: Fix NFS KConfig typos

Two issues were noticed after the NFS v4.0 KConfig changes were merged
upstream. First, the text of CONFIG_NFS_V4 should not encourage people
to select it if they are unsure. Second, the new CONFIG_NFS_V4_0 option
should default to "on" instead of "off" to avoid breaking people's
setups if they are using NFS v4.0.

Reported-by: Niklas Cassel <cassel@kernel.org>
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Fixes: 4e0269352534 ("NFS: Add a way to disable NFS v4.0 via KConfig")
Fixes: 7537db24806f ("NFS: Merge CONFIG_NFS_V4_1 with CONFIG_NFS_V4")
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>