libzfs: refresh property cache after inheriting userprop

This matches what happens when inheriting a system property

Consider the following program:
	int main() {
		void *zhp = libzfs_init();
		void *dataset = zfs_open(zhp, "zest/__test", 1);

		printf("before:");
		dump_nvlist(zfs_get_user_props(dataset), 2);
		printf("\n");

		zfs_prop_inherit(dataset, "xyz.nabijaczleweli:test", 0);
		printf("after:");
		dump_nvlist(zfs_get_user_props(dataset), 2);
		printf("\n");

		zfs_refresh_properties(dataset);
		printf("refreshed:");
		dump_nvlist(zfs_get_user_props(dataset), 2);
		printf("\n");
	}

And the output before:
	# zfs set xyz.nabijaczleweli:test=hehe zest/__test
	# ./a.out
	before:  xyz.nabijaczleweli:test:
	      value: 'hehe'
	      source: 'zest/__test'

	after:  xyz.nabijaczleweli:test:
	      value: 'hehe'
	      source: 'zest/__test'

	refreshed:

As compared to the output after:
	# zfs set xyz.nabijaczleweli:test=hehe zest/__test
	# ./a.out
	before:  xyz.nabijaczleweli:test:
	      value: 'hehe'
	      source: 'zest/__test'

	after:
	refreshed:

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11064
Closes #11911

lib/libzfs/libzfs_dataset.c

@@ -1955,6 +1955,7 @@ zfs_prop_inherit(zfs_handle_t *zhp, const char *propname, boolean_t received)
 		if (zfs_ioctl(zhp->zfs_hdl, ZFS_IOC_INHERIT_PROP, &zc) != 0)
 			return (zfs_standard_error(hdl, errno, errbuf));
 
+		(void) get_stats(zhp);
 		return (0);
 	}