RDMA/rtrs-srv: Fix memory leak of unfreed rtrs_srv_stats object

When closing a session, currently the rtrs_srv_stats object in the
closing session is freed by kobject release. But if it failed
to create a session by various reasons, it must free the rtrs_srv_stats
object directly because kobject is not created yet.

This problem is found by kmemleak as below:

1. One client machine maps /dev/nullb0 with session name 'bla':
root@test1:~# echo "sessname=bla path=ip:192.168.122.190 \
device_path=/dev/nullb0" > /sys/devices/virtual/rnbd-client/ctl/map_device

2. Another machine failed to create a session with the same name 'bla':
root@test2:~# echo "sessname=bla path=ip:192.168.122.190 \
device_path=/dev/nullb1" > /sys/devices/virtual/rnbd-client/ctl/map_device
-bash: echo: write error: Connection reset by peer

3. The kmemleak on server machine reported an error:
unreferenced object 0xffff888033cdc800 (size 128):
  comm "kworker/2:1", pid 83, jiffies 4295086585 (age 2508.680s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000a72903b2>] __alloc_sess+0x1d4/0x1250 [rtrs_server]
    [<00000000d1e5321e>] rtrs_srv_rdma_cm_handler+0xc31/0xde0 [rtrs_server]
    [<00000000bb2f6e7e>] cma_ib_req_handler+0xdc5/0x2b50 [rdma_cm]
    [<00000000e896235d>] cm_process_work+0x2d/0x100 [ib_cm]
    [<00000000b6866c5f>] cm_req_handler+0x11bc/0x1c40 [ib_cm]
    [<000000005f5dd9aa>] cm_work_handler+0xe65/0x3cf2 [ib_cm]
    [<00000000610151e7>] process_one_work+0x4bc/0x980
    [<00000000541e0f77>] worker_thread+0x78/0x5c0
    [<00000000423898ca>] kthread+0x191/0x1e0
    [<000000005a24b239>] ret_from_fork+0x3a/0x50

Fixes: 39c2d639

 ("RDMA/rtrs-srv: Set .release function for rtrs srv device during device init")
Link: https://lore.kernel.org/r/20210528113018.52290-18-jinpu.wang@ionos.com
Signed-off-by: Gioh Kim <gi-oh.kim@ionos.com>
Signed-off-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>