f2fs: move mnt_want_write_file after range check

This patch move mnt_want_write_file after range check,
it's needless to check arguments with it.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>