Fix compile-time warnings caused by duplicate struct typedefs

Some compiler/versions warn these typedefs according to openzfs#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 openzfs#16660 
Closes openzfs#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
 

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;
@@ -41,22 +43,20 @@ 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);
 
 __attribute__((malloc))
-abd_t *abd_alloc_from_pages(struct page **, unsigned long, uint64_t);
+struct abd *abd_alloc_from_pages(struct page **, unsigned long, uint64_t);
 
 #ifdef __cplusplus
 }