libceph: a per-osdc crush scratch buffer

With the addition of erasure coding support in the future, scratch
variable-length array in crush_do_rule_ary() is going to grow to at
least 200 bytes on average, on top of another 128 bytes consumed by
rawosd/osd arrays in the call chain.  Replace it with a buffer inside
struct osdmap and a mutex.  This shouldn't result in any contention,
because all osd requests were already serialized by request_mutex at
that point; the only unlocked caller was ceph_ioctl_get_dataloc().

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>

include/linux/ceph/osdmap.h

@@ -84,6 +84,9 @@ struct ceph_osdmap {
 	/* the CRUSH map specifies the mapping of placement groups to
 	 * the list of osds that store+replicate them. */
 	struct crush_map *crush;
+
+	struct mutex crush_scratch_mutex;
+	int crush_scratch_ary[CEPH_PG_MAX_SIZE * 3];
 };
 
 static inline void ceph_oid_set_name(struct ceph_object_id *oid,