style: remove unnecessary spaces in sa.h

Removed three unnecessary spaces in the definition of the
sa_attr_reg_t structure to improve code style consistency
and adhere to OpenZFS coding standards.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Peng Liu <littlenewton6@gmail.com>
Closes #16955