NFSv4: Fix a bug in nfs4_validate_mount_data()

The previous patch introduced a bug when copying the server address.

Also clarify a copy into the auth_flavours array: currently the two
size calculations are equivalent, but we may decide to change the size
of auth_flavors[] at some point.

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