Fix user properties output for zpool list

In zpool_get_user_prop, when called from zpool_expand_proplist and
collect_pool, we often have zpool_props present in zpool_handle_t equal
to NULL. This mostly happens when only one user property is requested
using zpool list -o <user_property>. Checking for this case and
correctly initializing the zpool_props field in zpool_handle_t fixes
this issue.

Interestingly, this issue does not occur if we query any other property
like name or guid along with a user property with -o flag because while
accessing properties like guid, zpool_prop_get_int is called which
checks for this case specifically and calls zpool_get_all_props.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes openzfs#16734

lib/libzfs/libzfs_pool.c

@@ -471,13 +471,15 @@ int
 zpool_get_userprop(zpool_handle_t *zhp, const char *propname, char *buf,
     size_t len, zprop_source_t *srctype)
 {
-	nvlist_t *nv, *nvl;
+	nvlist_t *nv;
 	uint64_t ival;
 	const char *value;
 	zprop_source_t source = ZPROP_SRC_LOCAL;
 
-	nvl = zhp->zpool_props;
-	if (nvlist_lookup_nvlist(nvl, propname, &nv) == 0) {
+	if (zhp->zpool_props == NULL)
+		zpool_get_all_props(zhp);
+
+	if (nvlist_lookup_nvlist(zhp->zpool_props, propname, &nv) == 0) {
 		if (nvlist_lookup_uint64(nv, ZPROP_SOURCE, &ival) == 0)
 			source = ival;
 		verify(nvlist_lookup_string(nv, ZPROP_VALUE, &value) == 0);