NFSD: Replace /proc/fs/nfsd/supported_krb5_enctypes with a symlink

Now that I've added a file under /proc/net/rpc that is managed by
the SunRPC's Kerberos mechanism, replace NFSD's
supported_krb5_enctypes file with a symlink to the new SunRPC proc
file, which contains exactly the same content.

Remarkably, commit b0b0c0a2 ("nfsd: add proc file listing
kernel's gss_krb5 enctypes") added the nfsd_supported_krb5_enctypes
file in 2011, but this file has never been documented in nfsd(7).

Tested-by: Scott Mayhew <smayhew@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>