mtd: fix: avoid race condition when accessing mtd->usecount

mtd: fix: avoid race condition when accessing mtd->usecount

On A MIPS 32-cores machine a BUG_ON was triggered because some acesses to
mtd->usecount were done without taking mtd_table_mutex.
kernel: Call Trace:
kernel: [<ffffffff80401818>] __put_mtd_device+0x20/0x50
kernel: [<ffffffff804086f4>] blktrans_release+0x8c/0xd8
kernel: [<ffffffff802577e0>] __blkdev_put+0x1a8/0x200
kernel: [<ffffffff802579a4>] blkdev_close+0x1c/0x30
kernel: [<ffffffff8022006c>] __fput+0xac/0x250
kernel: [<ffffffff80171208>] task_work_run+0xd8/0x120
kernel: [<ffffffff8012c23c>] work_notifysig+0x10/0x18
kernel:
kernel:
        Code: 2442ffff  ac8202d8  000217fe <00020336> dc820128  10400003
               00000000  0040f809  00000000
kernel: ---[ end trace 080fbb4579b47a73 ]---

Fixed by taking the mutex in blktrans_open and blktrans_release.

Note that this locking is already suggested in
include/linux/mtd/blktrans.h:

struct mtd_blktrans_ops {
...
	/* Called with mtd_table_mutex held; no race with add/remove */
	int (*open)(struct mtd_blktrans_dev *dev);
	void (*release)(struct mtd_blktrans_dev *dev);
...
};

But we weren't following it.

Originally reported by (and patched by) Zhang and Giuseppe,
independently. Improved and rewritten.

Cc: stable@vger.kernel.org
Reported-by: Zhang Xingcai <zhangxingcai@huawei.com>
Reported-by: Giuseppe Cantavenera <giuseppe.cantavenera.ext@nokia.com>
Tested-by: Giuseppe Cantavenera <giuseppe.cantavenera.ext@nokia.com>
Acked-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>