Call parent verify_permissions() for float_csr_t (#832)

We were not properly checking for Machine-level or Supervisor-level
CSR accessibility.

No functional change, but only because all float_csr_t have User-level
permissions today.

riscv/csrs.cc

@@ -1145,6 +1145,7 @@ float_csr_t::float_csr_t(processor_t* const proc, const reg_t addr, const reg_t
 }
 
 void float_csr_t::verify_permissions(insn_t insn, bool write) const {
+  masked_csr_t::verify_permissions(insn, write);
   require_fp;
   if (!proc->extension_enabled('F'))
     throw trap_illegal_instruction(insn.bits());