blkzone: deny destructive ioctls on busy blockdev

blkzone: deny destructive ioctls on busy blockdev

If a user submits a zone management ioctl from user-space, like a zone
reset and a file-system (like zonefs or f2fs) is mounted on the zoned
block device, the zone will get reset and the file-system's cached value
of the zone's write-pointer becomes invalid.

Subsequent writes to this zone from the file-system will result in
unaligned writes and the drive will error out.

Open the block device file in exclusive mode for submitting these ioctls.
If a file-system is mounted the kernel will return -EBUSY and we can't
continue issuing the ioctl.

[kzak@redhat.com: - remove unnecessary error message]

Reported-by: Coly Li <colyli@suse.de>
Cc: Damien Le Moal <Damien.LeMoal@wdc.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Karel Zak <kzak@redhat.com>