x86/fpu: Fix guest FPU state buffer allocation size

Ongoing work on an optimization to batch-preallocate vCPU state buffers
for KVM revealed a mismatch between the allocation sizes used in
fpu_alloc_guest_fpstate() and fpstate_realloc(). While the former
allocates a buffer sized to fit the default set of XSAVE features
in UABI form (as per fpu_user_cfg), the latter uses its ksize argument
derived (for the requested set of features) in the same way as the sizes
found in fpu_kernel_cfg, i.e. using the compacted in-kernel
representation.

The correct size to use for guest FPU state should indeed be the
kernel one as seen in fpstate_realloc(). The original issue likely
went unnoticed through a combination of UABI size typically being
larger than or equal to kernel size, and/or both amounting to the
same number of allocated 4K pages.

Fixes: 69f6ed1d14c6 ("x86/fpu: Provide infrastructure for KVM FPU cleanup")
Signed-off-by: Stanislav Spassov <stanspas@amazon.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250218141045.85201-1-stanspas@amazon.de

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