block/rnbd: Fix an IS_ERR() vs NULL check in find_or_create_sess()

The alloc_sess() function returns error pointers, it never returns NULL.

Fixes: f7a7a5c2 ("block/rnbd: client: main functionality")
Link: https://lore.kernel.org/r/20200519120347.GD42765@mwanda


Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>