rust_binder: shrink all_procs when deregistering processes

When a process is deregistered from the binder context, the all_procs
vector may have significant unused capacity. Add logic to shrink the
vector using a conservative strategy that prevents shrink-then-regrow
oscillation.

The shrinking strategy triggers when length drops below 1/4 of capacity,
and shrinks to twice the current length rather than to the exact length.
This provides hysteresis to avoid repeated reallocations when the process
count fluctuates.

The shrink operation uses GFP_KERNEL and is allowed to fail gracefully
since it is purely an optimization. The vector remains valid and
functional even if shrinking fails.

Suggested-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Shivam Kalra <shivamkalra98@zohomail.in>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260216-binder-shrink-vec-v3-v6-3-ece8e8593e53@zohomail.in
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>