x86/fault: Cast an argument to the proper address space in prefetch()

Commit in Fixes uses accessors based on the access mode, i.e., it
distinguishes its access if instr carries a user address or a kernel
address.

Since that commit, sparse complains about passing an argument without
__user annotation to get_user(), which expects a pointer of the __user
address space:

  arch/x86/mm/fault.c:152:29: warning: incorrect type in argument 1 (different address spaces)
  arch/x86/mm/fault.c:152:29:    expected void const volatile [noderef] __user *ptr
  arch/x86/mm/fault.c:152:29:    got unsigned char *[assigned] instr

Cast instr to __user when accessing user memory.

No functional change. No change in the generated object code.

  [ bp: Simplify commit message. ]

Fixes: 35f1c89b ("x86/fault: Fix AMD erratum #91 errata fixup for user code")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220201144055.5670-1-lukas.bulwahn@gmail.com