f2fs: relocate f2fs_precache_extents() in f2fs_swap_activate()

This patch exchangs position of f2fs_precache_extents() and
filemap_fdatawrite(), so that f2fs_precache_extents() can load
extent info after physical addresses of all data are fixed.

Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>