VMIO reads: enable for UFS

Move v_object creation earlier, so that VIRF_PGREAD is never set if
v_object is NULL. There is no much harm from instantiating v_object
when later check for append-only flags disallows open.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D25968