abd_os: split userspace and Linux kernel code

The Linux abd_os.c serves double-duty as the userspace scatter abd
implementation, by carrying an emulation of kernel scatterlists. This
commit lifts common and userspace-specific parts out into a separate
abd_os.c for libzpool.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16253

include/sys/abd.h

@@ -68,7 +68,9 @@ typedef struct abd {
 		} abd_scatter;
 		struct abd_linear {
 			void		*abd_buf;
+#if defined(__linux__) && defined(_KERNEL)
 			struct scatterlist *abd_sgl; /* for LINEAR_PAGE */
+#endif
 		} abd_linear;
 		struct abd_gang {
 			list_t abd_gang_chain;