null_alloc_dev() allocates memory for dev->badblocks, but cleanup
currently only occurs in the configfs release codepath, missing a number
of other places.

This bug was found running the blktests block/010 test, alongside
kmemleak:
rapido1:/blktests# ./check block/010
...
rapido1:/blktests# echo scan > /sys/kernel/debug/kmemleak
[  306.966708] kmemleak: 32 new suspected memory leaks (see /sys/kernel/debug/kmemleak)
rapido1:/blktests# cat /sys/kernel/debug/kmemleak
unreferenced object 0xffff88001f86d000 (size 4096):
  comm "modprobe", pid 231, jiffies 4294892415 (age 318.252s)
  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:
    [<ffffffff814b0379>] kmemleak_alloc+0x49/0xa0
    [<ffffffff810f180f>] kmem_cache_alloc+0x9f/0xe0
    [<ffffffff8124e45f>] badblocks_init+0x2f/0x60
    [<ffffffffa0019fae>] 0xffffffffa0019fae
    [<ffffffffa0021273>] nullb_device_badblocks_store+0x63/0x130 [null_blk]
    [<ffffffff810004cd>] do_one_initcall+0x3d/0x170
    [<ffffffff8109fe0d>] do_init_module+0x56/0x1e9
    [<ffffffff8109ebd7>] load_module+0x1c47/0x26a0
    [<ffffffff8109f819>] SyS_finit_module+0xa9/0xd0
    [<ffffffff814b4f60>] entry_SYSCALL_64_fastpath+0x13/0x94

Fixes: 2f54a613 ("nullb: badbblocks support")
Reviewed-by: Shaohua Li <shli@fb.com>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>