migration/multifd: fix destroyed mutex access in terminating multifd threads

migration/multifd: fix destroyed mutex access in terminating multifd threads

One multifd will lock all the other multifds' IOChannel mutex to inform them
to quit by setting p->quit or shutting down p->c. In this senario, if some
multifds had already been terminated and multifd_load_cleanup/multifd_save_cleanup
had destroyed their mutex, it could cause destroyed mutex access when trying
lock their mutex.

Here is the coredump stack:
    #0  0x00007f81a2794437 in raise () from /usr/lib64/libc.so.6
    #1  0x00007f81a2795b28 in abort () from /usr/lib64/libc.so.6
    #2  0x00007f81a278d1b6 in __assert_fail_base () from /usr/lib64/libc.so.6
    #3  0x00007f81a278d262 in __assert_fail () from /usr/lib64/libc.so.6
    #4  0x000055eb1bfadbd3 in qemu_mutex_lock_impl (mutex=0x55eb1e2d1988, file=<optimized out>, line=<optimized out>) at util/qemu-thread-posix.c:64
    #5  0x000055eb1bb4564a in multifd_send_terminate_threads (err=<optimized out>) at migration/ram.c:1015
    #6  0x000055eb1bb4bb7f in multifd_send_thread (opaque=0x55eb1e2d19f8) at migration/ram.c:1171
    #7  0x000055eb1bfad628 in qemu_thread_start (args=0x55eb1e170450) at util/qemu-thread-posix.c:502
    #8  0x00007f81a2b36df5 in start_thread () from /usr/lib64/libpthread.so.0
    #9  0x00007f81a286048d in clone () from /usr/lib64/libc.so.6

To fix it up, let's destroy the mutex after all the other multifd threads had
been terminated.

Signed-off-by: Jiahui Cen <cenjiahui@huawei.com>
Signed-off-by: Ying Fang <fangying1@huawei.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>