RDMA/rtrs: Remove a useless kfree()

'sess->rbufs' is known to be NULL here, so there is no point in kfree'ing
it.

Fixes: 6a98d71d ("RDMA/rtrs: client: main functionality")
Link: https://lore.kernel.org/r/9a57c9f837fa2c6f0070578a1bc4840688f62962.1628185335.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>