Allow tuning zfs_max_recordsize via loader tunable.  Tuning is NOT
recommended.

Requested by:	Slawa Olhovchenkov <slw zxy spb ru>
MFC after:	2 weeks


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274673 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f