dm-verity: avoid double increment of &use_bh_wq_enabled

dm-verity: avoid double increment of &use_bh_wq_enabled

verity_parse_opt_args is called twice, first with the only_modifier_opts,
first with only_modifier_opts == true and then with only_modifier_opts ==
false. Thus, the static branch &use_bh_wq_enabled was incremented twice
and the destructor verity_dtr would only decrement it once.

Fix tihs bug by only incrementing it on the first call, on the second
call, when v->use_bh_wq is true, do nothing.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Cc: stable@vger.kernel.org
Fixes: df326e7a0699 ("dm verity: allow optional args to alter primary args handling")