rds: Use nested-BH locking for rds_page_remainder

rds_page_remainder is a per-CPU variable and relies on disabled BH for its
locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
this data structure requires explicit locking.

Add a local_lock_t to the data structure and use
local_lock_nested_bh() for locking. This change adds only lockdep
coverage and does not alter the functional behaviour for !PREEMPT_RT.

Cc: Allison Henderson <[e-mail]>
Cc: [e-mail]
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/rds/page.c

···
 struct rds_page_remainder {
 	struct page	*r_page;
 	unsigned long	r_offset;
+	local_lock_t	bh_lock;
 };
 
-static
-DEFINE_PER_CPU_SHARED_ALIGNED(struct rds_page_remainder, rds_page_remainders);
+static DEFINE_PER_CPU_SHARED_ALIGNED(struct rds_page_remainder, rds_page_remainders) = {
+	.bh_lock = INIT_LOCAL_LOCK(bh_lock),
+};
 
 /**
  * rds_page_remainder_alloc - build up regions of a message.
···
 	}
 
 	local_bh_disable();
+	local_lock_nested_bh(&rds_page_remainders.bh_lock);
 	rem = this_cpu_ptr(&rds_page_remainders);
 
 	while (1) {
···
 		}
 
 		/* alloc if there is nothing for us to use */
+		local_unlock_nested_bh(&rds_page_remainders.bh_lock);
 		local_bh_enable();
 
 		page = alloc_page(gfp);
 
 		local_bh_disable();
+		local_lock_nested_bh(&rds_page_remainders.bh_lock);
 		rem = this_cpu_ptr(&rds_page_remainders);
 
 		if (!page) {
···
 		rem->r_offset = 0;
 	}
 
+	local_unlock_nested_bh(&rds_page_remainders.bh_lock);
 	local_bh_enable();
 out:
 	rdsdebug("bytes %lu ret %d %p %u %u\n", bytes, ret,