arm64/ptrace: Avoid uninitialised struct padding in fpr_set()

This patch adds an explicit __reserved[] field to user_fpsimd_state
to replace what was previously unnamed padding.

This ensures that data in this region are propagated across
assignment rather than being left possibly uninitialised at the
destination.

Cc: <stable@vger.kernel.org> # 3.7.x-
Fixes: 60ffc30d ("arm64: Exception handling")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <Will.Deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>