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/spl/spl-condvar.c

@@ -37,7 +37,7 @@
 #endif
 
 #define	MAX_HRTIMEOUT_SLACK_US	1000
-unsigned int spl_schedule_hrtimeout_slack_us = 0;
+static unsigned int spl_schedule_hrtimeout_slack_us = 0;
 
 static int
 param_set_hrtimeout_slack(const char *buf, zfs_kernel_param_t *kp)

module/os/linux/spl/spl-err.c

@@ -32,7 +32,7 @@
  * analysis and other such goodies.
  * But we would still default to the current default of not to do that.
  */
-unsigned int spl_panic_halt;
+static unsigned int spl_panic_halt;
 /* CSTYLED */
 module_param(spl_panic_halt, uint, 0644);
 MODULE_PARM_DESC(spl_panic_halt, "Cause kernel panic on assertion failures");

module/os/linux/spl/spl-generic.c

@@ -90,7 +90,7 @@ EXPORT_SYMBOL(p0);
  * and use them when in_interrupt() from linux/preempt_mask.h evaluates to
  * true.
  */
-void __percpu *spl_pseudo_entropy;
+static void __percpu *spl_pseudo_entropy;
 
 /*
  * spl_rand_next()/spl_rand_jump() are copied from the following CC-0 licensed

module/os/linux/spl/spl-kmem-cache.c

@@ -151,7 +151,7 @@ MODULE_PARM_DESC(spl_kmem_cache_kmem_threads,
 
 struct list_head spl_kmem_cache_list;   /* List of caches */
 struct rw_semaphore spl_kmem_cache_sem; /* Cache list lock */
-taskq_t *spl_kmem_cache_taskq;		/* Task queue for aging / reclaim */
+static taskq_t *spl_kmem_cache_taskq;   /* Task queue for aging / reclaim */
 
 static void spl_cache_shrink(spl_kmem_cache_t *skc, void *obj);