kprobes/x86: Consolidate insn decoder users for copying code

Consolidate x86 instruction decoder users on the path of
copying original code for kprobes.

Kprobes decodes the same instruction a maximum of 3 times when
preparing the instruction buffer:

 - The first time for getting the length of the instruction,
 - the 2nd for adjusting displacement,
 - and the 3rd for checking whether the instruction is boostable or not.

For each time, the actual decoding target address is slightly
different (1st is original address or recovered instruction buffer,
2nd and 3rd are pointing to the copied buffer), but all have
the same instruction.

Thus, this patch also changes the target address to the copied
buffer at first and reuses the decoded "insn" for displacement
adjusting and checking boostability.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David S . Miller <davem@davemloft.net>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ye Xiaolong <xiaolong.ye@intel.com>
Link: http://lkml.kernel.org/r/149076389643.22469.13151892839998777373.stgit@devbox
Signed-off-by: Ingo Molnar <mingo@kernel.org>