Protect Flymake's eager checks against commands like fill-paragraph

Protect Flymake's eager checks against commands like fill-paragraph

If flymake-start-syntax-check-on-newline is t, check should start as
soon as a newline is seen by after-change-functions.  But don't rush
it: since the buffer state might not be final, we might end up with
invalid diagnostic regions after some commands silently insert and
delete newlines (looking at you, fill-paragraph).

* lisp/progmodes/flymake.el (flymake-after-change-function): Pass
`deferred' to flymake--start-syntax-check.
(flymake--start-syntax-check): Take optional `deferred' arg.