x86/hyperv: Use current_stack_pointer to avoid asm() in hv_hvcrash_ctxt_save()

Use current_stack_pointer to avoid asm() when saving %rsp to the
crash context memory in hv_hvcrash_ctxt_save(). The new code is
more readable and results in exactly the same object file.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Long Li <longli@microsoft.com>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>