module: add stubs for within_module functions

Provide stubs for within_module_core(), within_module_init(), and
within_module() to prevent build errors when !CONFIG_MODULES.

Suggested-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kbuild test robot <lkp@intel.com>
Link: https://marc.info/?l=linux-mm&m=155384681109231&w=2


Signed-off-by: Tri Vo <trong@android.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>