freebsd: return EISDIR for read(2) on directories

This is arguably a change for internal consistency within OpenZFS, as the
Linux implementation will reject read(2) on directories with EISDIR. It's
not unreasonable for read(2) to do something here on FreeBSD, but we don't
currently copy out anything useful anyways so start rejecting it with the
appropriate error.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Kyle Evans <kevans@FreeBSD.org>
Closes #10338