scsi: target: core: Fix target_cmd_counter leak

In the Linux kernel, the following vulnerability has been resolved:

scsi: target: core: Fix target_cmd_counter leak

The target_cmd_counter struct allocated via target_alloc_cmd_counter() is
never freed, resulting in leaks across various transport types, e.g.:

unreferenced object 0xffff88801f920120 (size 96):
comm "sh", pid 102, jiffies 4294892535 (age 713.412s)
hex dump (first 32 bytes):
07 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 38 01 92 1f 80 88 ff ff ........8.......
backtrace:
[<00000000e58a6252>] kmalloc_trace+0x11/0x20
[<0000000043af4b2f>] target_alloc_cmd_counter+0x17/0x90 [target_core_mod]
[<000000007da2dfa7>] target_setup_session+0x2d/0x140 [target_core_mod]
[<0000000068feef86>] tcm_loop_tpg_nexus_store+0x19b/0x350 [tcm_loop]
[<000000006a80e021>] configfs_write_iter+0xb1/0x120
[<00000000e9f4d860>] vfs_write+0x2e4/0x3c0
[<000000008143433b>] ksys_write+0x80/0xb0
[<00000000a7df29b2>] do_syscall_64+0x42/0x90
[<0000000053f45fb8>] entry_SYSCALL_64_after_hwframe+0x6e/0xd8

Free the structure alongside the corresponding iscsit_conn / se_sess
parent.