riscv: avoid sending a SIGTRAP to a user thread trapped in WARN()

On RISC-V, when the kernel runs code on behalf of a user thread, and the
kernel executes a WARN() or WARN_ON(), the user thread will be sent
a bogus SIGTRAP.  Fix the RISC-V kernel code to not send a SIGTRAP when
a WARN()/WARN_ON() is executed.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
[paul.walmsley@sifive.com: fixed subject]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>