Fixes the appended use-after-free. The root cause is that
during tb invalidation we use CPU_FOREACH, and therefore
to safely free a vCPU we must wait for an RCU grace period
to elapse.

$ x86_64-linux-user/qemu-x86_64 tests/tcg/x86_64-linux-user/munmap-pthread
=================================================================
==1800604==ERROR: AddressSanitizer: heap-use-after-free on address 0x62d0005f7418 at pc 0x5593da6704eb bp 0x7f4961a7ac70 sp 0x7f4961a7ac60
READ of size 8 at 0x62d0005f7418 thread T2
    #0 0x5593da6704ea in tb_jmp_cache_inval_tb ../accel/tcg/tb-maint.c:244
    #1 0x5593da6704ea in do_tb_phys_invalidate ../accel/tcg/tb-maint.c:290
    #2 0x5593da670631 in tb_phys_invalidate__locked ../accel/tcg/tb-maint.c:306
    #3 0x5593da670631 in tb_invalidate_phys_page_range__locked ../accel/tcg/tb-maint.c:542
    #4 0x5593da67106d in tb_invalidate_phys_range ../accel/tcg/tb-maint.c:614
    #5 0x5593da6a64d4 in target_munmap ../linux-user/mmap.c:766
    #6 0x5593da6dba05 in do_syscall1 ../linux-user/syscall.c:10105
    #7 0x5593da6f564c in do_syscall ../linux-user/syscall.c:13329
    #8 0x5593da49e80c in cpu_loop ../linux-user/x86_64/../i386/cpu_loop.c:233
    #9 0x5593da6be28c in clone_func ../linux-user/syscall.c:6633
    #10 0x7f496231cb42 in start_thread nptl/pthread_create.c:442
    #11 0x7f49623ae9ff  (/lib/x86_64-linux-gnu/libc.so.6+0x1269ff)

0x62d0005f7418 is located 28696 bytes inside of 32768-byte region [0x62d0005f0400,0x62d0005f8400)
freed by thread T148 here:
    #0 0x7f49627b6460 in __interceptor_free ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:52
    #1 0x5593da5ac057 in cpu_exec_unrealizefn ../cpu.c:180
    #2 0x5593da81f851  (/home/cota/src/qemu/build/qemu-x86_64+0x484851)

Signed-off-by: Emilio Cota <cota@braap.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230111151628.320011-2-cota@braap.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230124180127.1881110-27-alex.bennee@linaro.org>