x86/sev: Fix kernel crash due to late update to read-only ghcb_version

A write-access violation page fault kernel crash was observed while running
cpuhotplug LTP testcases on SEV-ES enabled systems. The crash was
observed during hotplug, after the CPU was offlined and the process
was migrated to different CPU. setup_ghcb() is called again which
tries to update ghcb_version in sev_es_negotiate_protocol(). Ideally this
is a read_only variable which is initialised during booting.

Trying to write it results in a pagefault:

  BUG: unable to handle page fault for address: ffffffffba556e70
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0003) - permissions violation
  [ ...]
  Call Trace:
   <TASK>
   ? __die_body.cold+0x1a/0x1f
   ? __die+0x2a/0x35
   ? page_fault_oops+0x10c/0x270
   ? setup_ghcb+0x71/0x100
   ? __x86_return_thunk+0x5/0x6
   ? search_exception_tables+0x60/0x70
   ? __x86_return_thunk+0x5/0x6
   ? fixup_exception+0x27/0x320
   ? kernelmode_fixup_or_oops+0xa2/0x120
   ? __bad_area_nosemaphore+0x16a/0x1b0
   ? kernel_exc_vmm_communication+0x60/0xb0
   ? bad_area_nosemaphore+0x16/0x20
   ? do_kern_addr_fault+0x7a/0x90
   ? exc_page_fault+0xbd/0x160
   ? asm_exc_page_fault+0x27/0x30
   ? setup_ghcb+0x71/0x100
   ? setup_ghcb+0xe/0x100
   cpu_init_exception_handling+0x1b9/0x1f0

The fix is to call sev_es_negotiate_protocol() only in the BSP boot phase,
and it only needs to be done once in any case.

[ mingo: Refined the changelog. ]

Fixes: 95d33bfa ("x86/sev: Register GHCB memory when SEV-SNP is active")
Suggested-by: Tom Lendacky <thomas.lendacky@amd.com>
Co-developed-by: Bo Gan <bo.gan@broadcom.com>
Signed-off-by: Bo Gan <bo.gan@broadcom.com>
Signed-off-by: Ashwin Dayanand Kamat <ashwin.kamat@broadcom.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/1701254429-18250-1-git-send-email-kashwindayan@vmware.com