rust_binder: update Process::node_refs to use SpinLock

Unfortunately the current use of a mutex for this lock leads to priority
inversion. Traces have been observed where a process is trying to obtain
this mutex for 22ms, but it's unable to do so because the thread holding
the lock is scheduled out. Since this occurred on a UI thread, that is
an extremely long delay.

Code paths that might sleep under this lock have already been updated in
patches leading up to this one.

Reviewed-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260615-binder-noderefs-spin-v3-6-3235f5a3e0a0@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>