lowmem: remove unecessary usage of proc_dointvec_minmax()

As far as I can understand, using proc_dointvec_minmax() instead of
proc_dointvec only makes sense if you supply the min and/or max values in the
extra1 and extra2 fields of the struct.

Signed-off-by: Viktor Rosendahl <viktor.rosendahl@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>