Marking this as __packed was needed to get the alignment and offset of

Marking this as __packed was needed to get the alignment and offset of
members right.  However, it also said it was aligned(1), which meant
that gcc generated really bad code.  Mark this as aligned(4).  This
makes things a little faster on arm (a couple percent), but also saves
about 30k on the size of the kernel for arm.

I talked about doing this with bde, but didn't check with him before
the commit, so I'm hesitant say 'reviewed by: bde'.