rxrpc: Fix rxrpc_rotate_tx_rotate() to check there's something to rotate

rxrpc: Fix rxrpc_rotate_tx_rotate() to check there's something to rotate

Fix rxrpc_rotate_tx_rotate() to check that there's something in the
transmission buffer to be rotated before it attempts to rotate anything.

Fixes: b341a0263b1b ("rxrpc: Implement progressive transmission queue struct")
Link: https://sashiko.dev/#/patchset/20260618134802.2477777-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260624163819.3017002-12-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>