ZIL: Allow to replay blocks of any size.

There seems to be no reason for ZIL blocks to be limited by 128KB
other than replay code is written in such a way. This change does
not increase the limit yet, just removes the artificial limitation.

Avoided extra memcpy() may save us a second during replay.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Prakash Surya <prakash.surya@delphix.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #14910