xprtrdma: Fix Read chunk padding

commit 24abdf1be15c478e2821d6fc903a4a4440beff02 upstream.

When pad optimization is disabled, rpcrdma_convert_iovs still
does not add explicit XDR round-up padding to a Read chunk.

Commit 677eb17e ("xprtrdma: Fix XDR tail buffer marshalling")
incorrectly short-circuited the test for whether round-up padding
is needed that appears later in rpcrdma_convert_iovs.

However, if this is indeed a regular Read chunk (and not a
Position-Zero Read chunk), the tail iovec _always_ contains the
chunk's padding, and never anything else.

So, it's easy to just skip the tail when padding optimization is
enabled, and add the tail in a subsequent Read chunk segment, if
disabled.

Fixes: 677eb17e ("xprtrdma: Fix XDR tail buffer marshalling")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>