fork: fold legacy_clone_args_valid() into _do_fork()

This separate helper only existed to guarantee the mutual exclusivity of
CLONE_PIDFD and CLONE_PARENT_SETTID for legacy clone since CLONE_PIDFD
abuses the parent_tid field to return the pidfd. But we can actually handle
this uniformely thus removing the helper. For legacy clone we can detect
that CLONE_PIDFD is specified in conjunction with CLONE_PARENT_SETTID
because they will share the same memory which is invalid and for clone3()
setting the separate pidfd and parent_tid fields to the same memory is
bogus as well. So fold that helper directly into _do_fork() by detecting
this case.

Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: "Peter Zijlstra (Intel)" <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Christian Brauner <[email protected]>

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