f2fs: specify extent cache for read explicitly

[ Upstream commit 12607c1b ]

Let's descrbie it's read extent cache.

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>