device-dax/core: Fix memory leak when rmmod dax.ko

When I repeatedly modprobe and rmmod dax.ko, kmemleak report a
memory leak as follows:

unreferenced object 0xffff9a5588c05088 (size 8):
  comm "modprobe", pid 261, jiffies 4294693644 (age 42.063s)
...
  backtrace:
    [<00000000e007ced0>] kstrdup+0x35/0x70
    [<000000002ae73897>] kstrdup_const+0x3d/0x50
    [<000000002b00c9c3>] kvasprintf_const+0xbc/0xf0
    [<000000008023282f>] kobject_set_name_vargs+0x3b/0xd0
    [<00000000d2cbaa4e>] kobject_set_name+0x62/0x90
    [<00000000202e7a22>] bus_register+0x7f/0x2b0
    [<000000000b77792c>] 0xffffffffc02840f7
    [<000000002d5be5ac>] 0xffffffffc02840b4
    [<00000000dcafb7cd>] do_one_initcall+0x58/0x240
    [<00000000049fe480>] do_init_module+0x56/0x1e2
    [<0000000022671491>] load_module+0x2517/0x2840
    [<000000001a2201cb>] __do_sys_finit_module+0x9c/0xe0
    [<000000003eb304e7>] do_syscall_64+0x33/0x40
    [<0000000051c5fd06>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

When rmmod dax is executed, dax_bus_exit() is missing. This patch
can fix this bug.

Fixes: 9567da0b ("device-dax: Introduce bus + driver model")
Cc: <stable@vger.kernel.org>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Link: https://lore.kernel.org/r/20201201135929.66530-1-wanghai38@huawei.comSigned-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dax/super.c

@@ -752,6 +752,7 @@ static int __init dax_core_init(void)
 
 static void __exit dax_core_exit(void)
 {
+	dax_bus_exit();
 	unregister_chrdev_region(dax_devt, MINORMASK+1);
 	ida_destroy(&dax_minor_ida);
 	dax_fs_exit();