Pass the expected struct radix_node_head * to vfs_free_netcred.

No functional change.

struct radix_node_head's first element is rh so this was already
referring to the same address. It was likely an unintended
s/rnh/&rnh->rh/ change from r294706 as all other rnh_walktree() callers
pass the expected struct radix_node_head * rather than obscurely passing
the address of their first element.

Sponsored by: EMC / Isilon Storage Division