x86/asm/entry/64: Fold identical code paths

x86/asm/entry/64: Fold identical code paths

retint_kernel doesn't require %rcx to be pointing to thread info
(anymore?), and the code on the two alternative paths is - not
really surprisingly - identical.

Signed-off-by: Jan Beulich <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>