f2fs: preallocate blocks for encrypted file

This patch allow preallocates data blocks for buffered aio writes
in encrypted file.

Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: fix to avoid BUG_ON]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>