bug#58082: recent whitespace-mode regression for read-only buffers

[Kyle Meyer]

Before f47a5324f44 (whitespace: Redo BoB/EoB empty line highlighting,
2022-09-11, bug#37467), this snippet runs without an issue:

  (let ((b (generate-new-buffer "ws-read-only-test")))
    (with-current-buffer b
      (insert " ")
      (setq buffer-read-only t)
      (whitespace-mode 1))
    (pop-to-buffer b))

After that commit, activating whitespace mode fails:

  whitespace--update-bob-eob: Buffer is read-only: #<buffer ws-read-only-test>

Perhaps some of the additions in f47a5324f44 should let-bind
inhibit-read-only?