f2fs: fix to preallocate block only aligned to 4K

In write_begin(), we skip checking dnode block for preallocating block
when whole block needs to be updated since we preallocated its block in
f2fs_preallocate_blocks, for partial updated block, we will still try
to lock its node and do preallocation in write_begin(), so in
f2fs_preallocate_blocks we should not preallocate its block.

But previously, the calculation of preallocating block number is
incorrect, fix it.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: fix a bug]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>