f2fs: extract rb-tree operation infrastructure

rb-tree lookup/update functions are deeply coupled into extent cache
codes, it's very hard to reuse these basic functions, this patch
extracts common rb-tree operation infrastructure for latter reusing.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>