bdev: make blockdev_mnt static

bdev: make blockdev_mnt static

The blockdev_mnt are not used outside the file bdev.c, so the modification
is defined as static.

block/bdev.c:377:17: warning: symbol 'blockdev_mnt' was not declared. Should it be static?

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
jpg: Remove closes bugzilla link
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Fixes: 8f3a608827d1 ("bdev: open block device as files")
Tested-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20240614090345.655716-2-john.g.garry@oracle.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>