f2fs: let f2fs_precache_extents() traverses in file range

Rather than in range of [0, max_file_blocks()), since data after EOF
is alwasy zero, it's unnecessary to preload mapping info of the data.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>