riscv/futex: Optimize atomic cmpxchg

Remove redundant release/acquire barriers, optimizing the lr/sc sequence
to provide conditional RCsc synchronization, per the RVWMO.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20241113183321.491113-1-dave@stgolabs.net


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>