Removing ZERO_PAGE abd_alloc_zero_scatter

For MIPS architectures on Linux the ZERO_PAGE macro references
empty_zero_page, which is exported as a GPL symbol. The call to
ZERO_PAGE in abd_alloc_zero_scatter has been removed and a single
zero'd page is now allocated for each of the pages in abd_zero_scatter
in the kernel ABD code path.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
Closes #10428

include/sys/abd.h

@@ -42,7 +42,6 @@ typedef int abd_iter_func_t(void *buf, size_t len, void *priv);
 typedef int abd_iter_func2_t(void *bufa, void *bufb, size_t len, void *priv);
 
 extern int zfs_abd_scatter_enabled;
-extern abd_t *abd_zero_scatter;
 
 /*
  * Allocations and deallocations

include/sys/abd_impl.h

@@ -92,6 +92,8 @@ struct abd_iter {
 	struct scatterlist *iter_sg;	/* current sg */
 };
 
+extern abd_t *abd_zero_scatter;
+
 abd_t *abd_gang_get_offset(abd_t *, size_t *);
 
 /*