memblock: Make memblock_{add|remove|free|reserve}() return int and update prototypes

memblock_{add|remove|free|reserve}() return either 0 or -errno but had
long as return type.  Chage it to int.  Also, drop 'extern' from all
prototypes in memblock.h - they are unnecessary and used
inconsistently (especially if mm.h is included in the picture).

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Yinghai Lu <yinghai@kernel.org>