mm: fix section mismatch warnings

modpost had two cases hardcoded for mm/
Shift over to __init_refok and kill the
hardcoded function names in modpost.

This has the drawback that the functions
will always be kept no matter configuration.
With previous code the function were placed in
init section if configuration allowed it.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>