ffs: fix vn_read_from_obj() usage for PAGE_SIZE > block size

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