x86: Rename global percpu symbol dr7 to cpu_dr7

Percpu symbols now occupy the same namespace as other global
symbols and as such short global symbols without subsystem
prefix tend to collide with local variables.  dr7 percpu
variable used by x86 was hit by this. Rename it to cpu_dr7.

The rename also makes it more consistent with its fellow
cpu_debugreg percpu variable.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <20091125115856.GA17856@elte.hu>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>