f2fs: move internal functions into extent_cache.c

[ Upstream commit 3bac20a8 ]

No functional change.

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>