openrisc: traps: Don't send signals to kernel mode threads

openrisc: traps: Don't send signals to kernel mode threads

[ Upstream commit c88cfb5cea5f8f9868ef02cc9ce9183a26dcf20f ]

OpenRISC exception handling sends signals to user processes on floating
point exceptions and trap instructions (for debugging) among others.
There is a bug where the trap handling logic may send signals to kernel
threads, we should not send these signals to kernel threads, if that
happens we treat it as an error.

This patch adds conditions to die if the kernel receives these
exceptions in kernel mode code.

Fixes: 27267655c531 ("openrisc: Support floating point user api")
Signed-off-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>