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

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

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.