target/arm: Add missing 'return's after calling v7m_exception_taken()

In do_v7m_exception_exit(), we perform various checks as part of
performing the exception return.  If one of these checks fails, the
architecture requires that we take an appropriate exception on the
existing stackframe.  We implement this by calling
v7m_exception_taken() to set up to take the new exception, and then
immediately returning from do_v7m_exception_exit() without proceeding
any further with the unstack-and-exception-return process.

In a couple of checks that are new in v8.1M, we forgot the "return"
statement, with the effect that if bad code in the guest tripped over
these checks we would set up to take a UsageFault exception but then
blunder on trying to also unstack and return from the original
exception, with the probable result that the guest would crash.

Add the missing return statements.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210723162146.5167-3-peter.maydell@linaro.org

target/arm/m_helper.c

@@ -1554,6 +1554,7 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
                     qemu_log_mask(CPU_LOG_INT, "...taking UsageFault on existing "
                         "stackframe: NSACR prevents clearing FPU registers\n");
                     v7m_exception_taken(cpu, excret, true, false);
+                    return;
                 } else if (!cpacr_pass) {
                     armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE,
                                             exc_secure);
@@ -1561,6 +1562,7 @@ static void do_v7m_exception_exit(ARMCPU *cpu)
                     qemu_log_mask(CPU_LOG_INT, "...taking UsageFault on existing "
                         "stackframe: CPACR prevents clearing FPU registers\n");
                     v7m_exception_taken(cpu, excret, true, false);
+                    return;
                 }
             }
             /* Clear s0..s15, FPSCR and VPR */