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 <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
(cherry picked from commit a9851ea)

Co-authored-by: Tomohiro Kusumi <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>

Author: robn

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

@@ -33,6 +33,7 @@
 #include <sys/thread.h>
 #include <sys/rwlock.h>
 #include <sys/wait.h>
+#include <sys/kstat.h>
 
 #define	TASKQ_NAMELEN		31
 

include/os/linux/zfs/sys/abd_os.h

@@ -30,6 +30,8 @@
 extern "C" {
 #endif
 
+struct abd;
+
 struct abd_scatter {
 	uint_t		abd_offset;
 	uint_t		abd_nents;
@@ -41,19 +43,17 @@ struct abd_linear {
 	struct scatterlist *abd_sgl; /* for LINEAR_PAGE */
 };
 
-typedef struct abd abd_t;
-
 typedef int abd_iter_page_func_t(struct page *, size_t, size_t, void *);
-int abd_iterate_page_func(abd_t *, size_t, size_t, abd_iter_page_func_t *,
+int abd_iterate_page_func(struct abd *, size_t, size_t, abd_iter_page_func_t *,
     void *);
 
 /*
  * Linux ABD bio functions
  * Note: these are only needed to support vdev_classic. See comment in
  * vdev_disk.c.
  */
-unsigned int abd_bio_map_off(struct bio *, abd_t *, unsigned int, size_t);
-unsigned long abd_nr_pages_off(abd_t *, unsigned int, size_t);
+unsigned int abd_bio_map_off(struct bio *, struct abd *, unsigned int, size_t);
+unsigned long abd_nr_pages_off(struct abd *, unsigned int, size_t);
 
 #ifdef __cplusplus
 }