Reduce ARC fragmentaion threshold

[smh]

Summary

As ZFS can request up to SPA_MAXBLOCKSIZE memory block e.g. during zfs recv, update the threshold at which we start agressive reclamation to use SPA_MAXBLOCKSIZE (16M) instead of the lower zfs_max_recordsize which defaults to 1M.

This was reported here:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194513

[mav]

I have no objections. It may be too aggressive if fragmentation was somehow caused not by ZFS, but any way I don't know any other place in system which would bother to do this, while I know other places that suffer quietly from fragmentation.