NFS & NFSD: Update GSS dependencies

Geert reports that:
> On v6.2, "make ARCH=m68k defconfig" gives you
> CONFIG_RPCSEC_GSS_KRB5=m
> On v6.3, it became builtin, due to dropping the dependencies on
> the individual crypto modules.
>
> $ grep -E "CRYPTO_(MD5|DES|CBC|CTS|ECB|HMAC|SHA1|AES)" .config
> CONFIG_CRYPTO_AES=y
> CONFIG_CRYPTO_AES_TI=m
> CONFIG_CRYPTO_DES=m
> CONFIG_CRYPTO_CBC=m
> CONFIG_CRYPTO_CTS=m
> CONFIG_CRYPTO_ECB=m
> CONFIG_CRYPTO_HMAC=m
> CONFIG_CRYPTO_MD5=m
> CONFIG_CRYPTO_SHA1=m

This behavior is triggered by the "default y" in the definition of
RPCSEC_GSS.

The "default y" was added in 2010 by commit df486a25 ("NFS: Fix
the selection of security flavours in Kconfig"). However,
svc_gss_principal was removed in 2012 by commit 03a4e1f6


("nfsd4: move principal name into svc_cred"), so the 2010 fix is
no longer necessary. We can safely change the NFS_V4 and NFSD_V4
dependencies back to RPCSEC_GSS_KRB5 to get the nicer v6.2
behavior back.

Selecting KRB5 symbolically represents the true requirement here:
that all spec-compliant NFSv4 implementations must have Kerberos
available to use.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: dfe9a123

 ("SUNRPC: Enable rpcsec_gss_krb5.ko to be built without CRYPTO_DES")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>