arm64: kernel thread don't need to save fpsimd context.

kernel thread's default fpsimd state is zero. When fork a thread, if parent is kernel thread,
 and save hardware context to parent's fpsimd state, but this hardware context is user
process's context, because kernel thread don't use fpsimd, it will not introduce issue,
it add a little cost.

Change-Id: I292c0b328983b1ecaf642503183473c12bb1207b
Signed-off-by: Janet Liu <janet.liu@spreadtrum.com>
Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: 6eb6c80187c55b7f95683bc6502dccac54b95b92
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Se Wang (Patrick) Oh <sewango@codeaurora.org>