vn_read_from_obj() requires that all pages of a vnode (except the last
partial page) be either completely valid or completely invalid,
but for file systems with block size smaller than PAGE_SIZE,
partially valid pages may exist anywhere in the file.
Do not enable the vn_read_from_obj() path in this case.

Reviewed by:	mckusick, kib, markj
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D34836