dm-verity: fix a memory leak if some arguments are specified multiple times

If some of the arguments "check_at_most_once", "ignore_zero_blocks",
"use_fec_from_device", "root_hash_sig_key_desc" were specified more than
once on the target line, a memory leak would happen.

This commit fixes the memory leak. It also fixes error handling in
verity_verify_sig_parse_opt_args.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org