riscv: Ensure the value of FP registers in the core dump file is up to date

The value of FP registers in the core dump file comes from the
thread.fstate. However, kernel saves the FP registers to the thread.fstate
only before scheduling out the process. If no process switch happens
during the exception handling process, kernel will not have a chance to
save the latest value of FP registers to thread.fstate. It will cause the
value of FP registers in the core dump file may be incorrect. To solve this
problem, this patch force lets kernel save the FP register into the
thread.fstate if the target task_struct equals the current.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Fixes: b8c8a959

 ("RISC-V: Add FP register ptrace support for gdb.")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>