del_mtd_device() will call of_node_put() to mtd_get_of_node(mtd), which
is mtd->dev.of_node. However, memset(&mtd->dev, 0) is called before
of_node_put(). As the result, of_node_put() won't do anything in
del_mtd_device(), and causes the refcount leak.

del_mtd_device()
    memset(&mtd->dev, 0, sizeof(mtd->dev) # clear mtd->dev
    of_node_put()
        mtd_get_of_node(mtd) # mtd->dev is cleared, can't locate of_node
                             # of_node_put(NULL) won't do anything

Fix the error by caching the pointer of the device_node.

OF: ERROR: memory leak, expected refcount 1 instead of 2,
of_node_get()/of_node_put() unbalanced - destroy cset entry: attach
overlay node /spi/spi-sram@0
CPU: 3 PID: 275 Comm: python3 Tainted: G N 6.1.0-rc3+ #54
    0d8a1edddf51f172ff5226989a7565c6313b08e2
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
rel-1.15.0-0-g2dd4b9b3f840-prebuilt.qemu.org 04/01/2014
Call Trace:
<TASK>
    dump_stack_lvl+0x67/0x83
    kobject_get+0x155/0x160
    of_node_get+0x1f/0x30
    of_fwnode_get+0x43/0x70
    fwnode_handle_get+0x54/0x80
    fwnode_get_nth_parent+0xc9/0xe0
    fwnode_full_name_string+0x3f/0xa0
    device_node_string+0x30f/0x750
    pointer+0x598/0x7a0
    vsnprintf+0x62d/0x9b0
    ...
    cfs_overlay_release+0x30/0x90
    config_item_release+0xbe/0x1a0
    config_item_put+0x5e/0x80
    configfs_rmdir+0x3bd/0x540
    vfs_rmdir+0x18c/0x320
    do_rmdir+0x198/0x330
    __x64_sys_rmdir+0x2c/0x40
    do_syscall_64+0x37/0x90
    entry_SYSCALL_64_after_hwframe+0x63/0xcd

Fixes: 00596576 ("mtd: core: clear out unregistered devices a bit more")
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
[<miquel.raynal@bootlin.com>: Light reword of the commit log]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20221119063915.11108-1-shangxiaojing@huawei.com