zfs_dirty_data_max_max grows without bound as memory is installed

[locdtm]

As of #1696 (comment), zfs_dirty_data_max_max is dynamically set to a fraction of physmem * PAGESIZE where it used to be statically set to 4 GiB. As the comment notes, this change was intended to prevent overflow of zfs_dirty_data_max and zfs_dirty_data_max_max on 32-bit systems.

It's clear that these values need to shrink on 32-bit systems.

It's not clear that these values would need to grow on large 64-bit systems.

Ideally, zfs_dirty_data_max_max would be capped to 4 GB (as it was before).

[behlendorf]

@evujumenuk thanks for reminding us about this. Yes, this was an oversight and we should reinstate it which I've done in PR #6081. @tuxoko I believe you ran in to trouble with this when testing on a large memory machine, if you could review this that'd be great.