arm64: fpsimd: Avoid FPSIMD context leakage for the init task

The init task is started with thread_flags equal to 0, which means
that TIF_FOREIGN_FPSTATE is initially clear.

It is theoretically possible (if unlikely) that the init task could
reach userspace without ever being scheduled out.  If this occurs,
data left in the FPSIMD registers by the kernel could be exposed.

This patch fixes this anomaly by ensuring that the init task's
initial TIF_FOREIGN_FPSTATE is set.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Fixes: 005f78cd ("arm64: defer reloading a task's FPSIMD state to userland resume")
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>