knfsd: rpc: fix server-side wrapping of krb5i replies

It's not necessarily correct to assume that the xdr_buf used to hold the
server's reply must have page data whenever it has tail data.

And there's no need for us to deal with that case separately anyway.

Acked-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>