abd_os: remove redundant refcount creation for abd_children

Refcount creation for abd_zero_scatter->abd_children is redundant in
abd_alloc_zero_scatter, as it has been done in abd_init_struct.

In addition, abd_children is undefined when ZFS_DEBUG is disabled, the
reference of abd_children in abd_alloc_zero_scatter breaks build of
libzpool when ZFS_DEBUG is disabled.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: Ping Huang <huangping@smartx.com>
Closes #13429

module/os/linux/zfs/abd_os.c

@@ -619,7 +619,6 @@ abd_alloc_zero_scatter(void)
 	ABD_SCATTER(abd_zero_scatter).abd_offset = 0;
 	ABD_SCATTER(abd_zero_scatter).abd_nents = nr_pages;
 	abd_zero_scatter->abd_size = SPA_MAXBLOCKSIZE;
-	zfs_refcount_create(&abd_zero_scatter->abd_children);
 	ABD_SCATTER(abd_zero_scatter).abd_sgl = vmem_alloc(nr_pages *
 	    sizeof (struct scatterlist), KM_SLEEP);