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

module/os/freebsd/spl/spl_sysevent.c

@@ -114,7 +114,7 @@ log_sysevent(nvlist_t *event)
 		}
 		case DATA_TYPE_STRING:
 		{
-			char *value;
+			const char *value;
 
 			(void) nvpair_value_string(elem, &value);
 			sbuf_printf(sb, " %s=%s", nvpair_name(elem), value);
@@ -180,7 +180,7 @@ log_sysevent(nvlist_t *event)
 		}
 		case DATA_TYPE_STRING_ARRAY:
 		{
-			char **strarr;
+			const char **strarr;
 			uint_t ii, nelem;
 
 			(void) nvpair_value_string_array(elem, &strarr, &nelem);

module/os/freebsd/zfs/zfs_znode.c

@@ -1690,7 +1690,7 @@ zfs_create_fs(objset_t *os, cred_t *cr, nvlist_t *zplprops, dmu_tx_t *tx)
 	while ((elem = nvlist_next_nvpair(zplprops, elem)) != NULL) {
 		/* For the moment we expect all zpl props to be uint64_ts */
 		uint64_t val;
-		char *name;
+		const char *name;
 
 		ASSERT3S(nvpair_type(elem), ==, DATA_TYPE_UINT64);
 		val = fnvpair_value_uint64(elem);

module/os/linux/zfs/zfs_znode.c

@@ -1866,7 +1866,7 @@ zfs_create_fs(objset_t *os, cred_t *cr, nvlist_t *zplprops, dmu_tx_t *tx)
 	while ((elem = nvlist_next_nvpair(zplprops, elem)) != NULL) {
 		/* For the moment we expect all zpl props to be uint64_ts */
 		uint64_t val;
-		char *name;
+		const char *name;
 
 		ASSERT(nvpair_type(elem) == DATA_TYPE_UINT64);
 		VERIFY(nvpair_value_uint64(elem, &val) == 0);