Fix sa.c to build on FreeBSD again. (#16403)

Fix multiple build errors on FreeBSD.

The main reason is, that the variable 'dxattr_obj' is used
uninitialized within the start of the 'out label'.

Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>