[XFS] fix unaligned access in readdir

This patch should fix the issue seen on Alpha with unaligned accesses in
the new readdir code. By aligning each dirent to sizeof(u64) we'll avoid
unaligned accesses. To make doubly sure we're not hitting problems also
rearrange struct hack_dirent to avoid holes.

SGI-PV: 975411
SGI-Modid: xfs-linux-melb:xfs-kern:30302a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>