null_blk: fix kmemleak by releasing references to fault configfs items

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

null_blk: fix kmemleak by releasing references to fault configfs items

When CONFIG_BLK_DEV_NULL_BLK_FAULT_INJECTION is enabled, the null-blk
driver sets up fault injection support by creating the timeout_inject,
requeue_inject, and init_hctx_fault_inject configfs items as children
of the top-level nullbX configfs group.

However, when the nullbX device is removed, the references taken to
these fault-config configfs items are not released. As a result,
kmemleak reports a memory leak, for example:

unreferenced object 0xc00000021ff25c40 (size 32):
comm "mkdir", pid 10665, jiffies 4322121578
hex dump (first 32 bytes):
69 6e 69 74 5f 68 63 74 78 5f 66 61 75 6c 74 5f init_hctx_fault_
69 6e 6a 65 63 74 00 88 00 00 00 00 00 00 00 00 inject..........
backtrace (crc 1a018c86):
__kmalloc_node_track_caller_noprof+0x494/0xbd8
kvasprintf+0x74/0xf4
config_item_set_name+0xf0/0x104
config_group_init_type_name+0x48/0xfc
fault_config_init+0x48/0xf0
0xc0080000180559e4
configfs_mkdir+0x304/0x814
vfs_mkdir+0x49c/0x604
do_mkdirat+0x314/0x3d0
sys_mkdir+0xa0/0xd8
system_call_exception+0x1b0/0x4f0
system_call_vectored_common+0x15c/0x2ec

Fix this by explicitly releasing the references to the fault-config
configfs items when dropping the reference to the top-level nullbX
configfs group.