mm: Add ptep_try_set() for lockless empty-slot installs

Add ptep_try_set(ptep, new_pte): atomically set *ptep to new_pte iff it is
currently pte_none(). Returns true on success, false if the slot was already
populated or the arch has no implementation.

The intended caller is the upcoming bpf_arena kernel-side fault recovery
path. The install runs from a page fault that can be nested under locks
held by the faulting kernel caller (e.g. a BPF program holding
raw_res_spin_lock_irqsave on its arena's spinlock), so trylock-and-retry
would A-A deadlock. Lock-free cmpxchg is the only viable option, which
constrains this helper to special kernel page tables where concurrent
writers cooperate via atomic accessors.

The generic version in <linux/pgtable.h> returns false. x86 and arm64
override with try_cmpxchg-based implementations on the underlying pteval.
Other architectures get the false stub - the callers there already fall
through to oops.

v2: Rename to ptep_try_set(). Tighten kerneldoc. (David, Alexei)
v3: Note that strict-zero cmpxchg is narrower than pte_none(). (Andrea)

Suggested-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Cc: David Hildenbrand <david@kernel.org>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Link: https://lore.kernel.org/r/20260522172219.1423324-2-tj@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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