dm integrity: fix gcc 5 warning

This commit fixes gcc 5 warning "logical not is only applied to the left
hand side of comparison"

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: fb098768 ("dm-integrity: introduce the Inline mode")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>