uprobes/x86: Fold prepare_fixups() into arch_uprobe_analyze_insn()

No functional changes, preparation.

Shift the code from prepare_fixups() to arch_uprobe_analyze_insn()
with the following modifications:

	- Do not call insn_get_opcode() again, it was already called
	  by validate_insn_bits().

	- Move "case 0xea" up. This way "case 0xff" can fall through
	  to default case.

	- change "case 0xff" to use the nested "switch (MODRM_REG)",
	  this way the code looks a bit simpler.

	- Make the comments look consistent.

While at it, kill the initialization of rip_rela_target_address and
->fixups, we can rely on kzalloc(). We will add the new members into
arch_uprobe, it would be better to assume that everything is zero by
default.

TODO: cleanup/fix the mess in validate_insn_bits() paths:

	- validate_insn_64bits() and validate_insn_32bits() should be
	  unified.

	- "ifdef" is not used consistently; if good_insns_64 depends
	  on CONFIG_X86_64, then probably good_insns_32 should depend
	  on CONFIG_X86_32/EMULATION

	- the usage of mm->context.ia32_compat looks wrong if the task
	  is TIF_X32.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Reviewed-by: Jim Keniston <jkenisto@us.ibm.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>