nvpair: Constify string functions

After addressing coverity complaints involving `nvpair_name()`, the
compiler started complaining about dropping const. This lead to a rabbit
hole where not only `nvpair_name()` needed to be constified, but also
`nvpair_value_string()`, `fnvpair_value_string()` and a few other static
functions, plus variable pointers throughout the code. The result became
a fairly big change, so it has been split out into its own patch.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14612

tests/zfs-tests/cmd/draid.c

@@ -1267,7 +1267,7 @@ draid_merge_impl(nvlist_t *allcfgs, const char *srcfilename, int *mergedp)
 		uint64_t nv_worst_ratio;
 		uint64_t allcfg_worst_ratio;
 		nvlist_t *cfg, *allcfg;
-		char *key;
+		const char *key;
 
 		switch (nvpair_type(elem)) {
 		case DATA_TYPE_NVLIST:

tests/zfs-tests/cmd/ereports.c

@@ -62,7 +62,7 @@ static const char *const criteria_name[] = {
 static void
 print_ereport_line(nvlist_t *nvl)
 {
-	char *class;
+	const char *class;
 	int last = CRITERIA_NAMES_COUNT - 1;
 
 	/*
@@ -81,7 +81,7 @@ print_ereport_line(nvlist_t *nvl)
 		nvpair_t *nvp;
 		uint32_t i32 = 0;
 		uint64_t i64 = 0;
-		char *str = NULL;
+		const char *str = NULL;
 
 		if (nvlist_lookup_nvpair(nvl, criteria_name[i], &nvp) != 0) {
 			/* print a proxy for optional criteria */

tests/zfs-tests/cmd/nvlist_to_lua.c

@@ -39,7 +39,7 @@ nvlist_equal(nvlist_t *nvla, nvlist_t *nvlb)
 	 */
 	for (nvpair_t *pair = nvlist_next_nvpair(nvla, NULL);
 	    pair != NULL; pair = nvlist_next_nvpair(nvla, pair)) {
-		char *key = nvpair_name(pair);
+		const char *key = nvpair_name(pair);
 
 		if (!nvlist_exists(nvlb, key))
 			return (B_FALSE);