x86, dumpstack: Fix unused variable warning

x86, dumpstack: Fix unused variable warning

In dump_stack function, bp isn't used anymore, which is introduced by
commit c9b45feab2e239fcedeb8467494974fd8bf8ef0e. This patch removes bp
completely.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Cc: Soeren Sandmann <sandmann@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: H. Peter Anvin <hpa@zytor.com>
LKML-Reference: <AANLkTik9U_Z0WSZ7YjrykER_pBUfPDdgUUmtYx=R74nL@mail.gmail.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>