mbuf: fix 64-bit address alignment in 32-bit builds

mbuf: fix 64-bit address alignment in 32-bit builds

On i686 builds, the uin64_t type is 64-bits in size but is aligned to
32-bits only. This causes mbuf fields for rearm_data to not be 16-byte
aligned on 32-bit builds, which causes errors with some vector PMDs which
expect the rearm data to be aligned as on 64-bit.

Given that we cannot use the extra space in the data structures anyway, as
it's already used on 64-bit builds, we can just force alignment of the
physical address in the mbuf to 8-bytes in all cases. This has no effect on
64-bit systems, but fixes the updated PMDs on 32-bit.

Fixes: f4356d7ca168 ("net/i40e: eliminate mbuf write on rearm")
Fixes: f160666a1073 ("net/ixgbe: eliminate mbuf write on rearm")

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>