f2fs: refactor extent_cache to support for read and more

[ Upstream commit e7547dac ]

This patch prepares extent_cache to be ready for addition.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: 043d2d00 ("f2fs: factor out victim_entry usage from general rb_tree use")
Signed-off-by: Sasha Levin <sashal@kernel.org>