Revert "rw_semaphore: remove up/down_read_non_owner"

This reverts commit 11b80f459a.

Bcache needs rw semaphores for cache coherency in writeback mode -
writes have to take a read lock on a per cache device rw sem, and
release it when the bio completes.

But since this is for bios it's naturally not in the context of the
process that originally took the lock.

Signed-off-by: Kent Overstreet <koverstreet@google.com>
CC: Christoph Hellwig <hch@infradead.org>
CC: David Howells <dhowells@redhat.com>

kernel/rwsem.c

@@ -126,6 +126,15 @@ void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
 
 EXPORT_SYMBOL(_down_write_nest_lock);
 
+void down_read_non_owner(struct rw_semaphore *sem)
+{
+	might_sleep();
+
+	__down_read(sem);
+}
+
+EXPORT_SYMBOL(down_read_non_owner);
+
 void down_write_nested(struct rw_semaphore *sem, int subclass)
 {
 	might_sleep();
@@ -136,6 +145,13 @@ void down_write_nested(struct rw_semaphore *sem, int subclass)
 
 EXPORT_SYMBOL(down_write_nested);
 
+void up_read_non_owner(struct rw_semaphore *sem)
+{
+	__up_read(sem);
+}
+
+EXPORT_SYMBOL(up_read_non_owner);
+
 #endif