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

[kib]

Summary

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 by:    Andrew Gierth  <[e-mail]>

[andrew_tao173.riddles.org.uk]

Tested this the same way as I tested the original report; no failures detected. The approach looks reasonable to me.