RISC-V: selftests: cbo: Ensure asm operands match constraints

RISC-V: selftests: cbo: Ensure asm operands match constraints

The 'i' constraint expects a constant operand, which fn and its
constant derivative MK_CBO(fn) are, but passing fn through a function
as a parameter and using a local variable for MK_CBO(fn) allow the
compiler to lose sight of that when no optimization is done. Use
a macro instead of a function and skip the local variable to ensure
the compiler uses constants, matching the asm constraints.

Reported-by: Yunhui Cui <cuiyunhui@bytedance.com>
Closes: https://lore.kernel.org/all/20240117082514.42967-1-cuiyunhui@bytedance.com
Fixes: a29e2a48afe3 ("RISC-V: selftests: Add CBO tests")
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20240117130933.57514-2-ajones@ventanamicro.com
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>