Fix race in flush_old_exec

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ThreadSanitizer: data-race in flush_old_exec

Write of size 4 by thread T568 (K779):
 [<     inlined    >] flush_old_exec+0x2ac/0xed0 de_thread /fs/exec.c:994
 [<ffffffff8124a97c>] flush_old_exec+0x2ac/0xed0 /fs/exec.c:1066
 [<ffffffff812c7ed7>] load_elf_binary+0x4d7/0x2940 /fs/binfmt_elf.c:722
 [<ffffffff8124a383>] search_binary_handler+0x143/0x2f0 /fs/exec.c:1378
 [<     inlined    >] do_execve_common.isra.29+0x810/0xa40 exec_binprm /fs/exec.c:1415
 [<ffffffff8124c180>] do_execve_common.isra.29+0x810/0xa40 /fs/exec.c:1512
 [<ffffffff8124c3dd>] do_execve+0x2d/0x40 /fs/exec.c:1554
 [<ffffffff810a578b>] ____call_usermodehelper+0x23b/0x290 /kernel/kmod.c:242
 [<ffffffff81dea0bc>] ret_from_fork+0x7c/0xb0 /arch/x86/kernel/entry_64.S:347
DBG: cpu = ffff88041fc96030

Previous read of size 4 by thread T248 (K777):
 [<     inlined    >] wait_consider_task+0xc7/0x1740 eligible_child /kernel/exit.c:936
 [<ffffffff81082d37>] wait_consider_task+0xc7/0x1740 /kernel/exit.c:1302
 [<     inlined    >] do_wait+0x1ec/0x400 do_wait_thread /kernel/exit.c:1419
 [<ffffffff8108459c>] do_wait+0x1ec/0x400 /kernel/exit.c:1488
 [<     inlined    >] SyS_wait4+0xca/0x160 SYSC_wait4 /kernel/exit.c:1615
 [<ffffffff81084d2a>] SyS_wait4+0xca/0x160 /kernel/exit.c:1584
 [<ffffffff810a59be>] wait_for_helper+0x5e/0x90 /kernel/kmod.c:303
 [<ffffffff81dea0bc>] ret_from_fork+0x7c/0xb0 /arch/x86/kernel/entry_64.S:347
DBG: cpu = 0

DBG: addr: ffff8801161297e8
DBG: first offset: 0, second offset: 0
DBG: T568 clock: {T568: 7638, T248: 5479}
DBG: T248 clock: {T248: 5528}
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