kselftest/arm64: Don't open code SVE_PT_SIZE() in fp-ptrace

kselftest/arm64: Don't open code SVE_PT_SIZE() in fp-ptrace

In fp-trace when allocating a buffer to write SVE register data we open
code the addition of the header size to the VL depeendent register data
size, which lead to an underallocation bug when we cut'n'pasted the code
for FPSIMD format writes. Use the SVE_PT_SIZE() macro that the kernel
UAPI provides for this.

Fixes: b84d2b27954f ("kselftest/arm64: Test FPSIMD format data writes via NT_ARM_SVE in fp-ptrace")
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250812-arm64-fp-trace-macro-v1-1-317cfff986a5@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>