rust: devres: ensure revocation is complete before device finishes unbinding

Now that the revocation Completion is in place, also address the
symmetric case. When Devres::drop() wins the is_available swap and the
devres callback loses, the callback returns to devres_release_all()
without waiting. This means device unbinding can complete while
Devres::drop() is still executing drop_in_place() on another CPU, which
is a problem if T's destructor accesses device state.

Make the synchronization bidirectional. Whichever side performs
drop_in_place() signals the Completion, and the other side waits.

This does not reintroduce the nested Devres deadlock fixed by commit
ba268514ea14 ("rust: devres: fix race condition due to nesting"),
because that deadlock was caused by drop waiting for the release
callback to return (the old 'devm' Completion). Here, both sides only
wait for drop_in_place() to finish, which completes within the current
call chain. The Arc<Inner<T>> keeps the Inner allocation alive
independently.

Cc: stable@vger.kernel.org
Fixes: ba268514ea14 ("rust: devres: fix race condition due to nesting")
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260628200304.2365598-1-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

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