Reduce ARC fragmentation threshold

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.

PR: 194513
Reviewed by: avg, mav
MFC after: 1 month
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D10012