f2fs: check position in move range ioctl

When the move range ioctl is used, check the input and output position and
ensure that it is a non-negative value. Without this check
f2fs_get_dnode_of_data may hit a memmory bug.

Signed-off-by: Dan Robertson <dan@dlrobertson.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>