MIPS: Remove struct mm_context_t fp_mode_switching field

The fp_mode_switching field in struct mm_context_t was left unused by
commit 8c8d953c2800 ("MIPS: Schedule on CPUs we need to lose FPU for a
mode switch") in v4.19, with nothing modifying its value & nothing
waiting on it having any particular value after that commit. Remove the
unused field & the one remaining reference to it.

Signed-off-by: Paul Burton <[e-mail]>

arch/mips/include/asm/mmu.h

···
 typedef struct {
 	u64 asid[NR_CPUS];
 	void *vdso;
-	atomic_t fp_mode_switching;
 
 	/* lock to be held whilst modifying fp_bd_emupage_allocmap */
 	spinlock_t bd_emupage_lock;

arch/mips/kernel/process.c

···
 	/*
 	 * This is icky, but we use this to simply ensure that all CPUs have
 	 * context switched, regardless of whether they were previously running
-	 * kernel or user code. This ensures that no CPU currently has its FPU
-	 * enabled, or is about to attempt to enable it through any path other
-	 * than enable_restore_fp_context() which will wait appropriately for
-	 * fp_mode_switching to be zero.
+	 * kernel or user code. This ensures that no CPU that a mode-switching
+	 * program may execute on keeps its FPU enabled (& in the old mode)
+	 * throughout the mode switch.
 	 */
 	return 0;
 }
···
 	for_each_cpu_and(cpu, &process_cpus, cpu_online_mask)
 		work_on_cpu(cpu, prepare_for_fp_mode_switch, NULL);
 	put_online_cpus();
-
-	wake_up_var(&task->mm->context.fp_mode_switching);
 
 	return 0;
 }