drop ZUT_OBJ, zfs unit testing driver never materialzied in freebsd

MFC after:	5 days
zfs: make zfs_map_page / zfs_unmap_page public

MFC after:	15 days
zfs: add dmu_write_pages variant for freebsd

The freebsd variant of dmu_write_pages is hidden under _KERNEL
to avoid needlessly pulling in vm_page_t declaration.
Besides, this function seems to be useless for ZFS userland counterpart.

MFC after:	15 days
zfs: add zfs_freebsd_putpages

this should be more optimal than writing pages one-by-one via zfs_write ->
update_pages in the case of multi-page putpages call

MFC after:	16 days