btrfs: increase output size for LOGICAL_INO_V2 ioctl

Build-server workloads have hundreds of references per file after dedup.
Multiply by a few snapshots and we quickly exhaust the limit of 2730
references per extent that can fit into a 64K buffer.

Raise the limit to 16M to be consistent with other btrfs ioctls
(e.g. TREE_SEARCH_V2, FILE_EXTENT_SAME).

To minimize surprising userspace behavior, apply this change only to
the LOGICAL_INO_V2 ioctl.

Signed-off-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
Reviewed-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
Tested-by: Hans van Kranenburg <hans.van.kranenburg@mendix.com>
Signed-off-by: David Sterba <dsterba@suse.com>