zdb: Fix handling of nul termination in symlink targets

The SA attribute containing the symlink target does not include a nul
terminator, so when printing the target zdb would sometimes include
garbage at the end of the string.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #13482