f2fs: move sanity checking of cp into get_valid_checkpoint

>From the function name of get_valid_checkpoint, it seems to return
the valid cp or NULL for caller to check. If no valid one is found,
f2fs_fill_super will print the err log. But if get_valid_checkpoint
get one valid(the return value indicate that it's valid, however actually
it is invalid after sanity checking), then print another similar err
log. That seems strange. Let's keep sanity checking inside the procedure
of geting valid cp. Another improvement we gained from this move is
that even the large volume is supported, we check the cp in advanced
to skip the following procedure if failing the sanity checking.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>