f2fs: fix redundant block allocation

commit c040ff9d69fd1d782fe577ba9e35c1f5798158ae upstream.

In direct_IO path of f2fs_file_write_iter(),
1. f2fs_preallocate_blocks(F2FS_GET_BLOCK_PRE_DIO)
   -> allocate LBA X
2. f2fs_direct_IO()
   -> return 0;

Then,
f2fs_write_data_page() will allocate another LBA X+1.

This makes EIO triggered by HM-SMR.

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