x86/fpu/xstate: Fix inconsistencies in guest FPU xfeatures

Guest FPUs manage vCPU FPU states. They are allocated via
fpu_alloc_guest_fpstate() and are resized in fpstate_realloc() when XFD
features are enabled.

Since the introduction of guest FPUs, there have been inconsistencies in
the kernel buffer size and xfeatures:

 1. fpu_alloc_guest_fpstate() uses fpu_user_cfg since its introduction. See:

    69f6ed1d14c6 ("x86/fpu: Provide infrastructure for KVM FPU cleanup")
    36487e6228c4 ("x86/fpu: Prepare guest FPU for dynamically enabled FPU features")

 2. __fpstate_reset() references fpu_kernel_cfg to set storage attributes.

 3. fpu->guest_perm uses fpu_kernel_cfg, affecting fpstate_realloc().

A recent commit in the tip:x86/fpu tree partially addressed the inconsistency
between (1) and (3) by using fpu_kernel_cfg for size calculation in (1),
but left fpu_guest->xfeatures and fpu_guest->perm still referencing
fpu_user_cfg:

  https://lore.kernel.org/all/20250218141045.85201-1-stanspas@amazon.de/

  1937e18cc3cf ("x86/fpu: Fix guest FPU state buffer allocation size")

The inconsistencies within fpu_alloc_guest_fpstate() and across the
mentioned functions cause confusion.

Fix them by using fpu_kernel_cfg consistently in fpu_alloc_guest_fpstate(),
except for fields related to the UABI buffer. Referencing fpu_kernel_cfg
won't impact functionalities, as:

 1. fpu_guest->perm is overwritten shortly in fpu_init_guest_permissions()
    with fpstate->guest_perm, which already uses fpu_kernel_cfg.

 2. fpu_guest->xfeatures is solely used to check if XFD features are enabled.
    Including supervisor xfeatures doesn't affect the check.

Fixes: 36487e6228c4 ("x86/fpu: Prepare guest FPU for dynamically enabled FPU features")
Suggested-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Chao Gao <chao.gao@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Link: https://lore.kernel.org/r/20250317140613.1761633-1-chao.gao@intel.com

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