mtd: core: fix possible resource leak in init_mtd()

I got the error report while inject fault in init_mtd():

sysfs: cannot create duplicate filename '/devices/virtual/bdi/mtd-0'
Call Trace:
 <TASK>
 dump_stack_lvl+0x67/0x83
 sysfs_warn_dup+0x60/0x70
 sysfs_create_dir_ns+0x109/0x120
 kobject_add_internal+0xce/0x2f0
 kobject_add+0x98/0x110
 device_add+0x179/0xc00
 device_create_groups_vargs+0xf4/0x100
 device_create+0x7b/0xb0
 bdi_register_va.part.13+0x58/0x2d0
 bdi_register+0x9b/0xb0
 init_mtd+0x62/0x171 [mtd]
 do_one_initcall+0x6c/0x3c0
 do_init_module+0x58/0x222
 load_module+0x268e/0x27d0
 __do_sys_finit_module+0xd5/0x140
 do_syscall_64+0x37/0x90
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
 </TASK>
kobject_add_internal failed for mtd-0 with -EEXIST, don't try to register
	things with the same name in the same directory.
Error registering mtd class or bdi: -17

If init_mtdchar() fails in init_mtd(), mtd_bdi will not be unregistered,
as a result, we can't load the mtd module again, to fix this by calling
bdi_unregister(mtd_bdi) after out_procfs label.

Fixes: 445caaa20c ("mtd: Allocate bdi objects dynamically")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221024065109.2050705-1-cuigaosheng1@huawei.com

drivers/mtd/mtdcore.c

@@ -2489,6 +2489,7 @@ static int __init init_mtd(void)
 out_procfs:
 	if (proc_mtd)
 		remove_proc_entry("mtd", NULL);
+	bdi_unregister(mtd_bdi);
 	bdi_put(mtd_bdi);
 err_bdi:
 	class_unregister(&mtd_class);