Fix compile-time warnings caused by duplicate struct typedefs

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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #16660 
Closes #16665

include/os/linux/spl/sys/taskq.h

@@ -38,8 +38,7 @@
 #include <sys/rwlock.h>
 #include <sys/wait.h>
 #include <sys/wmsum.h>
-
-typedef struct kstat_s kstat_t;
+#include <sys/kstat.h>
 
 #define	TASKQ_NAMELEN		31