LoongArch64: avoid SGEMM LA464 ncopy fallthrough from N2 to N1

[wxpppp]

Problem

The .L_N2 tail path handles a remaining 2-column block. After that path completes, the original code could directly jump or fall through into .L_N1.

However, .L_N1 handles a remaining 1-column block, so entering .L_N1 should depend on N & 1.

The original code used M & 1:

.L_N2_M1:
    andi      I,     M,    0x01
    beq       I,     ZERO, .L_N1

M & 1 only indicates whether the current 2-column block has one remaining row. It does not indicate whether there is one remaining column.

This can cause the kernel to process an extra column after .L_N2, leading to an out-of-bounds read when the column tail contains 2 but not 1.

For example:

N % 8 == 2
N % 8 == 6

Fix

Add an .L_N2_END block and check N & 1 before falling through into .L_N1.

.L_N2_M1:
    andi      I,     M,    0x01
    beq       I,     ZERO, .L_N2_END
    ...

.L_N2_END:
    andi      J,     N,   0x01
    beq       ZERO,  J,   .L_N0

.align 5
.L_N1:

[martin-frbg]

Thank you