FROMGIT: f2fs: preload extent_cache for POSIX_FADV_WILLNEED

This patch tries to preload extent_cache given POSIX_FADV_WILLNEED, which is
more useful for generic usecases.

Bug: 301585673
Bug: 296299682
Fixes: f43b021e ("FROMGIT: f2fs: preload extent_cache for POSIX_FADV_WILLNEED")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 3e729e50
 https://git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)
Change-Id: I11702b172270b143209407d06aa0338f19f19b43