This patch adds a mount option 'extent_cache' in f2fs.
It is try to use a rb-tree based extent cache to cache more mapping information
with less memory if this option is set, otherwise we will use the original one
extent info cache.
Suggested-by: Changman Lee <cm224.lee@samsung.com> Signed-off-by: Chao Yu <chao2.yu@samsung.com> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>