after calling fork() in the test_prctl_fork_exec() function,
the global variable ksm_full_scans_fd is initialized to 0
in the child process upon entering the main function
of ./ksm_functional_tests.
In the function call chain test_child_ksm() ->
__mmap_and_merge_range -> ksm_merge-> ksm_get_full_scans,
start_scans = ksm_get_full_scans() will return an error.
Therefore, the value of ksm_full_scans_fd needs to be
initialized before calling test_child_ksm in the child process.
Signed-off-by: aigourensheng <shechenglong001@xxxxxxxxx>
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