mtd: fix lock hierarchy in deregister_mtd_blktrans

There is a lock hierarchy of major_names_lock --> mtd_table_mutex. One
existing chain is as follows:

1. major_names_lock --> loop_ctl_mutex (when blk_request_module calls
loop_probe)

2. loop_ctl_mutex --> bdev->bd_mutex (when loop_control_ioctl calls
loop_remove, which then calls del_gendisk)

3. bdev->bd_mutex --> mtd_table_mutex (when blkdev_get_by_dev calls
__blkdev_get, which then calls blktrans_open)

Since unregister_blkdev grabs the major_names_lock, we need to call it
outside the critical section for mtd_table_mutex, otherwise we invert
the lock hierarchy.

Reported-by: Hillf Danton <[email protected]>
Signed-off-by: Desmond Cheong Zhi Xi <[email protected]>
Signed-off-by: Miquel Raynal <[email protected]>
Link: https://lore.kernel.org/linux-mtd/[email protected]

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