taskstats: use better ifdef for alignment

Commit 4be2c95d ("taskstats: pad taskstats netlink response for aligment
issues on ia64") added a null field to align the taskstats structure but
the discussion centered around ia64.  The issue exists on other platforms
with inefficient unaligned access and adding them piecemeal would be an
unmaintainable mess.

This patch uses Dave Miller's suggestion of using a combination of
CONFIG_64BIT && !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to determine
whether alignment is needed.

Note that this will cause breakage on those platforms with applications
like iotop which had hard-coded offsets into the packet to access the
taskstats structure.

The message seen on systems without the alignment fixes looks like: kernel
unaligned access to 0xe000023879dca9bc, ip=0xa000000100133d10

The addresses may vary but resolve to locations inside __delayacct_add_tsk.

iotop makes what I'd call unreasonable assumptions about the contents of a
netlink genetlink packet containing generic ...