common/verity: use FSV_BLOCK_SIZE by default

common/verity: use FSV_BLOCK_SIZE by default

Make _fsv_enable() and _fsv_sign() default to FSV_BLOCK_SIZE if no block
size is explicitly specified, so that the individual tests don't have to
do this themselves.  This overrides the fsverity-utils default of 4096
bytes, or the page size in older versions of fsverity-utils, both of
which may differ from FSV_BLOCK_SIZE.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>