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;

module/os/linux/zfs/zfs_debug.c

@@ -35,7 +35,7 @@ typedef struct zfs_dbgmsg {
 
 static procfs_list_t zfs_dbgmsgs;
 static uint_t zfs_dbgmsg_size = 0;
-uint_t zfs_dbgmsg_maxsize = 4<<20; /* 4MB */
+static uint_t zfs_dbgmsg_maxsize = 4<<20; /* 4MB */
 
 /*
  * Internal ZFS debug messages are enabled by default.

module/os/linux/zfs/zvol_os.c

@@ -114,7 +114,7 @@ struct zvol_state_os {
 	boolean_t use_blk_mq;
 };
 
-taskq_t *zvol_taskq;
+static taskq_t *zvol_taskq;
 static struct ida zvol_ida;
 
 typedef struct zv_request_stack {