vdev_disk: rename existing functions to vdev_classic_*

This is just renaming the existing functions we're about to replace and
grouping them together to make the next commits easier to follow.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #15533
Closes #15588
(cherry picked from commit f3b85d706b)

include/sys/abd.h

@@ -220,6 +220,8 @@ void abd_fini(void);
 
 /*
  * Linux ABD bio functions
+ * Note: these are only needed to support vdev_classic. See comment in
+ * vdev_disk.c.
  */
 #if defined(__linux__) && defined(_KERNEL)
 unsigned int abd_bio_map_off(struct bio *, abd_t *, unsigned int, size_t);