open(2) has supports direct IO. When opening a file in the loopback
FS, writes to the underlying would fail, because the FUSE server does
not position the bytes to write (which come directly after the WriteIn
header) on a 512-byte boundary.

Fix this by allocating the input buffer with such that the first
content byte is aligned.

Add a test.

Fixes #328.

Change-Id: Ib7ad24aff673413ab8db4112d0b12143c2654617