Narrow scope of modification hook renabling in org-src fontification

Narrow scope of modification hook renabling in org-src fontification

Modification hooks should be enabled while modifying text in the
org-src temp buffer, but in 2017-01-29 "Call modification hooks in
org-src fontify buffers" the hooks were enabled also for modifications
to the original org buffer.  This causes fontification errors when
combined with certain packages, as reported in
http://lists.gnu.org/archive/html/emacs-orgmode/2017-03/msg00420.html.

* lisp/org/org-src.el (org-src-font-lock-fontify-block): Reduce scope
of inhibit-modification-hooks let-binding.