zdb: replace label_t to zdb_label_t for reduce collisions

with builds on illumos based platform we can see build issue
because label_t has been redefined.

for reduce build issues on others platforms we should rename
label_t to zdb_label_t.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Igor Kozhukhov <igor@dilos.org>
Closes #8397