Reapply "arm64: fpsimd: Implement lazy restore for kernel mode FPSIMD"

This reverts commit b8995a18.

Ard managed to reproduce the dm-crypt corruption problem and got to the
bottom of it, so re-apply the problematic patch in preparation for
fixing things properly.

Cc: stable@vger.kernel.org
Signed-off-by: Will Deacon <will@kernel.org>