Fix declarations of non-global variables

This patch inserts the `static` keyword to non-global variables,
which where found by the analysis tool smatch.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13970

module/os/linux/zfs/abd_os.c

@@ -132,7 +132,7 @@ static abd_stats_t abd_stats = {
 	{ "scatter_sg_table_retry",		KSTAT_DATA_UINT64 },
 };
 
-struct {
+static struct {
 	wmsum_t abdstat_struct_size;
 	wmsum_t abdstat_linear_cnt;
 	wmsum_t abdstat_linear_data_size;