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: 296299682
Suggested-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit a3edf9f0bbb798f6c39b2bd467ee2d7fe425a0f8
 https://git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git dev)
Change-Id: Ie4abc8b670658373dc0d4b8cae29990b6fb8bf9e