vnode read(2)/write(2): acquire rangelock regardless of do_vn_io_fault()

To ensure atomicity of reads against parallel writes and truncates,
vnode lock was not enough at least since introduction of vn_io_fault().
That code only take rangelock when it was possible that vn_read() and
vn_write() could drop the vnode lock.

At least since the introduction of VOP_READ_PGCACHE() which generally
does not lock the vnode at all, rangelocks become required even
for filesystems that do not need vn_io_fault() workaround. For
instance, tmpfs.

PR: 272678
Analyzed and reviewed by: Andrew Gierth <andrew@tao11.riddles.org.uk>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D41158