kprobes/x86: Mark 2 bytes NOP as boostable

Currently, x86 kprobes is unable to boost 2 bytes nop like:

  nopl 0x0(%rax,%rax,1)

which is 0x0f 0x1f 0x44 0x00 0x00.

Such nops have exactly 5 bytes to hold a relative jmp
instruction. Boosting them should be obviously safe.

This patch enable boosting such nops by simply updating
twobyte_is_boostable[] array.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: <lizefan@huawei.com>
Link: http://lkml.kernel.org/r/1423532045-41049-1-git-send-email-wangnan0@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>