riscv: Make __fstate_clean() work correctly.

Make the __fstate_clean() function correctly set the
state of sstatus.FS in pt_regs to SR_FS_CLEAN.

Fixes: 7db91e57 ("RISC-V: Task implementation")
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[paul.walmsley@sifive.com: expanded "Fixes" commit ID]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/include/asm/switch_to.h

@@ -16,7 +16,7 @@ extern void __fstate_restore(struct task_struct *restore_from);
 
 static inline void __fstate_clean(struct pt_regs *regs)
 {
-	regs->sstatus |= (regs->sstatus & ~(SR_FS)) | SR_FS_CLEAN;
+	regs->sstatus = (regs->sstatus & ~SR_FS) | SR_FS_CLEAN;
 }
 
 static inline void fstate_off(struct task_struct *task,