rds: Acquire per-CPU pointer within BH disabled section

rds: Acquire per-CPU pointer within BH disabled section

rds_page_remainder_alloc() obtains the current CPU with get_cpu() while
disabling preemption. Then the CPU number is used to access the per-CPU
data structure via per_cpu().

This can be optimized by relying on local_bh_disable() to provide a
stable CPU number/ avoid migration and then using this_cpu_ptr() to
retrieve the data structure.

Cc: Allison Henderson <allison.henderson@oracle.com>
Cc: linux-rdma@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20250512092736.229935-15-bigeasy@linutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>