compat: add LINUX_BACKPORT() for prefixing symbols

compat: add LINUX_BACKPORT() for prefixing symbols

Ben Hutchings <ben@decadent.org.uk> notes that "compat"
is already taken as a prefix for symbols and while this
is only slightly true in practice its best we avoid any
future issues.

Others in the past have noted issues with symbols exported
by backporting effort to conflict with other symbols that
might be preferred by the running kernel. In the worst
case scenerio we'd have the same subsystems with two eras
with two sets of drivers using a subystem from an era each.
This patch doesn't address that but tries to address the
namespace conflict by compat itself. The best alternative I
was hoping for was to use core module symbol namespaces but
after reviewing that effort introduced in 2007
by Andi Kleen [0] I see in the end Rusty Russell nack'd
these patches [1] so we're left with dealing with symbol
renaming.

Define LINUX_BACKPORT() to be used to allow us to rename
symbols with a backport_ prefix. The the cleanest, but
its a start.

[0] http://thread.gmane.org/gmane.linux.network/78674
[1] http://article.gmane.org/gmane.linux.kernel/606885

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>