ZAP: Some cleanups/micro-optimizations

• Remove custom zap_memset(), use regular memset().
• Use PANIC() instead of opaque cmn_err(CE_PANIC).
• Provide entry parameter to zap_leaf_rehash_entry().
• Reduce branching in zap_leaf_array_create() inner loop.
• Remove signedness where it should not be.

Should be no function changes.

Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #15976