rxrpc: Carry call state out of locked section in rxrpc_rotate_tx_window()

Carry the call state out of the locked section in rxrpc_rotate_tx_window()
rather than sampling it afterwards.  This is only used to select tracepoint
data, but could have changed by the time we do the tracepoint.

Signed-off-by: David Howells <dhowells@redhat.com>