f2fs: allocate the extent_cache by default

[ Upstream commit 72840ccc ]

Let's allocate it to remove the runtime complexity.

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>