Revert "rw_semaphore: remove up/down_read_non_owner"

Revert "rw_semaphore: remove up/down_read_non_owner"

This reverts commit ddedac27e78d37320f2eb13625991b39115c6571.

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>