dm-crypt: stop constraining max_segment_size to PAGE_SIZE

This change effectively reverts commit 586b286 ("dm crypt:
constrain crypt device's max_segment_size to PAGE_SIZE") and relies on
block core's late bio-splitting to ensure that dm-crypt's encryption
bios are split accordingly if they exceed the underlying device's
limits (e.g. max_segment_size).

Commit 586b286 was applied as a 4.3 fix for the benefit of
stable@ kernels 4.0+ just after block core's late bio-splitting was
introduced in 4.3 with commit 54efd50 ("block: make
generic_make_request handle arbitrarily sized bios"). Given block
core's late bio-splitting it is past time that dm-crypt make use of
it.

Also, given the recent need to revert meaningful progress that was
attempted during the 6.9 merge window (see commit bff4b74 Revert
"dm: use queue_limits_set") this change allows DM core to safely make
use of queue_limits_set() without risk of breaking dm-crypt on NVMe.
Though it should be noted this commit isn't a prereq for reinstating
DM core's use of queue_limits_set() because blk_validate_limits() was
made less strict with commit b561ea5 ("block: allow device to
have both virt_boundary_mask and max segment size").

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: Mike Snitzer

drivers/md/dm-crypt.c

@@ -1656,8 +1656,8 @@ static void crypt_free_buffer_pages(struct crypt_config *cc, struct bio *clone);
 
 /*
  * Generate a new unfragmented bio with the given size
- * This should never violate the device limitations (but only because
- * max_segment_size is being constrained to PAGE_SIZE).
+ * This should never violate the device limitations (but if it did then block
+ * core should split the bio as needed).
  *
  * This function may be called concurrently. If we allocate from the mempool
  * concurrently, there is a possibility of deadlock. For example, if we have
@@ -3717,14 +3717,6 @@ static void crypt_io_hints(struct dm_target *ti, struct queue_limits *limits)
 {
 	struct crypt_config *cc = ti->private;
 
-	/*
-	 * Unfortunate constraint that is required to avoid the potential
-	 * for exceeding underlying device's max_segments limits -- due to
-	 * crypt_alloc_buffer() possibly allocating pages for the encryption
-	 * bio that are not as physically contiguous as the original bio.
-	 */
-	limits->max_segment_size = PAGE_SIZE;
-
 	limits->logical_block_size =
 		max_t(unsigned int, limits->logical_block_size, cc->sector_size);
 	limits->physical_block_size =