riscv, lib: Fix Zbb strncmp

The Zbb optimized strncmp has two parts; a fast path that does XLEN/8B
per iteration, and a slow that does one byte per iteration.

The idea is to compare aligned XLEN chunks for most of strings, and do
the remainder tail in the slow path.

The Zbb strncmp has two issues in the fast path:

Incorrect remainder handling (wrong compare): Assume that the string
length is 9. On 64b systems, the fast path should do one iteration,
and one iteration in the slow path. Instead, both were done in the
fast path, which lead to incorrect results. An example:

  strncmp("/dev/vda", "/dev/", 5);

Correct by changing "bgt" to "bge".

Missing NULL checks in the second string: This could lead to incorrect
results for:

  strncmp("/dev/vda", "/dev/vda\0", 8);

Correct by adding an additional check.

Fixes: b6fcdb19 ("RISC-V: add zbb support to string functions")
Suggested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20230228184211.1585641-1-bjorn@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>