vdev_disk: try harder to ensure IO alignment rules

It seems out our notion of "properly" aligned IO was incomplete. In
particular, dm-crypt does its own splitting, and assumes that a logical
block will never cross an order-0 page boundary (ie, the physical page
size, not compound size). This effectively means that it needs to be
possible to split a BIO at any page or block size boundary and have it
work correctly.

This updates the alignment check function to enforce these rules (to the
extent possible).

Our response to misaligned data is to make some new allocation that is
properly aligned, and copy the data into it. It turns out that
linearising (via abd_borrow_buf()) is not enough, because we allocate eg
4K blocks from a general purpose slab, and so may receive (or already
have) a 4K block that crosses pages.

So instead, we allocate a new ABD, which is guaranteed to be aligned
properly to block sizes, and then copy everything into it, and back out
on the way back.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16687 #16631 #15646 #15533 #14533