dm-integrity: Avoid divide by zero in table status in Inline mode

In Inline mode, the journal is unused, and journal_sectors is zero.

Calculating the journal watermark requires dividing by journal_sectors,
which should be done only if the journal is configured.

Otherwise, a simple table query (dmsetup table) can cause OOPS.

This bug did not show on some systems, perhaps only due to
compiler optimization.

On my 32-bit testing machine, this reliably crashes with the following:

 : Oops: divide error: 0000 [#1] PREEMPT SMP
 : CPU: 0 UID: 0 PID: 2450 Comm: dmsetup Not tainted 6.14.0-rc2+ #959
 : EIP: dm_integrity_status+0x2f8/0xab0 [dm_integrity]
 ...

Signed-off-by: Milan Broz <gmazyland@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: fb098768 ("dm-integrity: introduce the Inline mode")
Cc: stable@vger.kernel.org # 6.11+