arm64: add abstractions for FPSIMD state manipulation

There are two tacit assumptions in the FPSIMD handling code that will no longer
hold after the next patch that optimizes away some FPSIMD state restores:
. the FPSIMD registers of this CPU contain the userland FPSIMD state of
  task 'current';
. when switching to a task, its FPSIMD state will always be restored from
  memory.

This patch adds the following functions to abstract away from straight FPSIMD
register file saves and restores:
- fpsimd_preserve_current_state -> ensure current's FPSIMD state is saved
- fpsimd_update_current_state -> replace current's FPSIMD state

Where necessary, the signal handling and fork code are updated to use the above
wrappers instead of poking into the FPSIMD registers directly.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

arch/arm64/kernel/signal32.c

@@ -219,7 +219,7 @@ static int compat_preserve_vfp_context(struct compat_vfp_sigframe __user *frame)
 	 * Note that this also saves V16-31, which aren't visible
 	 * in AArch32.
 	 */
-	fpsimd_save_state(fpsimd);
+	fpsimd_preserve_current_state();
 
 	/* Place structure header on the stack */
 	__put_user_error(magic, &frame->magic, err);
@@ -282,11 +282,8 @@ static int compat_restore_vfp_context(struct compat_vfp_sigframe __user *frame)
 	 * We don't need to touch the exception register, so
 	 * reload the hardware state.
 	 */
-	if (!err) {
-		preempt_disable();
-		fpsimd_load_state(&fpsimd);
-		preempt_enable();
-	}
+	if (!err)
+		fpsimd_update_current_state(&fpsimd);
 
 	return err ? -EFAULT : 0;
 }