This fixes to support dio having unaligned buffers as buffered writes.

xfs_io -f -d -c "pwrite 0 512" $testfile
 -> okay

xfs_io -f -d -c "pwrite 1 512" $testfile
 -> EINVAL
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>