RDMA/rtrs-srv: fix memory leak by missing kobject free

RDMA/rtrs-srv: fix memory leak by missing kobject free

kmemleak reported an error as below:

  unreferenced object 0xffff8880674b7640 (size 64):
    comm "kworker/4:1H", pid 113, jiffies 4296403507 (age 507.840s)
    hex dump (first 32 bytes):
      69 70 3a 31 39 32 2e 31 36 38 2e 31 32 32 2e 31  ip:192.168.122.1
      31 30 40 69 70 3a 31 39 32 2e 31 36 38 2e 31 32  10@ip:192.168.12
    backtrace:
      [<0000000054413611>] kstrdup+0x2e/0x60
      [<0000000078e3120a>] kobject_set_name_vargs+0x2f/0xb0
      [<00000000ca2be3ee>] kobject_init_and_add+0xb0/0x120
      [<0000000062ba5e78>] rtrs_srv_create_sess_files+0x14c/0x314 [rtrs_server]
      [<00000000b45b7217>] rtrs_srv_info_req_done+0x5b1/0x800 [rtrs_server]
      [<000000008fc5aa8f>] __ib_process_cq+0x94/0x100 [ib_core]
      [<00000000a9599cb4>] ib_cq_poll_work+0x32/0xc0 [ib_core]
      [<00000000cfc376be>] process_one_work+0x4bc/0x980
      [<0000000016e5c96a>] worker_thread+0x78/0x5c0
      [<00000000c20b8be0>] kthread+0x191/0x1e0
      [<000000006c9c0003>] ret_from_fork+0x3a/0x50

It is caused by the not-freed kobject of rtrs_srv_sess.  The kobject
embedded in rtrs_srv_sess has ref-counter 2 after calling
process_info_req(). Therefore it must call kobject_put twice.  Currently
it calls kobject_put only once at rtrs_srv_destroy_sess_files because
kobject_del removes the state_in_sysfs flag and then kobject_put in
free_sess() is not called.

This patch moves kobject_del() into free_sess() so that the kobject of
rtrs_srv_sess can be freed. And also this patch adds the missing call of
sysfs_remove_group() to clean-up the sysfs directory.

Fixes: 9cb837480424 ("RDMA/rtrs: server: main functionality")
Link: https://lore.kernel.org/r/20210212134525.103456-4-jinpu.wang@cloud.ionos.com
Signed-off-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>