Hi,
There is a bug that turn off semantic-mode it only restores current buffer's imenu-create-index-function but actually turning on semantic-mode had changed each buffer's variable.
Reproduce steps:
1) Open two files (eg /tmp/a.c, /tmp/b.c) in two buffers; 
2) Turn on semantic-mode, it will change the imenu-create-index-function value for each buffer;
3) Turn off semantic-mode, it will restore current buffer's value (eg: b.c), but another one (eg: a.c) won't restored.

When turning on semantic-mode, it applies changes to each buffer, 
 1048         (dolist (b (buffer-list))
 1049           (with-current-buffer b
 1050             (semantic-new-buffer-fcn))))

But turning off semantic does not restore the var for each buffer, only restores current buffer.

This patch will fix the issue. Please help review it. Thanks.
Best Regards, Lin