Fix compile-time warnings caused by duplicate struct typedefs (#16880)

Some compiler/versions warn these typedefs according to #16660.

The platform specific header sys/abd_os.h shouldn't define or use abd_t,
as it's defined in its non-platform specific consumer sys/abd.h.
Do the same as what FreeBSD header does.

Original-patch-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Rob Norris <robn@despairlabs.com>
(cherry picked from commit a9851ea3dd6af6f789e221f2ccd7ad43561eff81)

Co-authored-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>