libceph: add spinlock around osd->o_requests

In a later patch, we're going to need to search for a request in
the rbtree, but taking the o_mutex is inconvenient as we already
hold the con mutex at the point where we need it.

Add a new spinlock that we take when inserting and erasing entries from
the o_requests tree. Search of the rbtree can be done with either the
mutex or the spinlock, but insertion and removal requires both.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>