staging: lustre: Remove extern keyword from function prototypes

A function prototype is always a declaration and thus has "extern" prepended by default.

Signed-off-by: Ashley Smith <ashley@eclipso.ch>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>