linux(4): Add const qualifier to the value parameter of function handle_string()

The content that value point to is not going to be altered by function
handle_string().

MFC after: 1 week

(cherry picked from commit 68c890b4433d3655c7df91cc43f89f4d6a8b35e4)
(cherry picked from commit 1b3f88bc809526cc7cd22c88b4fdaadf1a2e5e75)