netfilter: xtables: make XT_ALIGN() usable in exported headers by exporting __ALIGN_KERNEL()

XT_ALIGN() was rewritten through ALIGN() by commit 42107f50
"netfilter: xtables: symmetric COMPAT_XT_ALIGN definition".
ALIGN() is not exported in userspace headers, which created compile problem for tc(8)
and will create problem for iptables(8).

We can't export generic looking name ALIGN() but we can export less generic
__ALIGN_KERNEL() (suggested by Ben Hutchings).
Google knows nothing about __ALIGN_KERNEL().

COMPAT_XT_ALIGN() changed for symmetry.

Reported-by: Andreas Henriksson <andreas@fatal.se>
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>