net/mlx5: fix a -Wstringop-truncation warning

net/mlx5: fix a -Wstringop-truncation warning

In file included from ./arch/powerpc/include/asm/paca.h:15,
                 from ./arch/powerpc/include/asm/current.h:13,
                 from ./include/linux/thread_info.h:21,
                 from ./include/asm-generic/preempt.h:5,
                 from ./arch/powerpc/include/generated/asm/preempt.h:1,
                 from ./include/linux/preempt.h:78,
                 from ./include/linux/spinlock.h:51,
                 from ./include/linux/wait.h:9,
                 from ./include/linux/completion.h:12,
                 from ./include/linux/mlx5/driver.h:37,
                 from
drivers/net/ethernet/mellanox/mlx5/core/lib/eq.h:6,
                 from
drivers/net/ethernet/mellanox/mlx5/core/diag/fw_tracer.c:33:
In function 'strncpy',
    inlined from 'mlx5_fw_tracer_save_trace' at
drivers/net/ethernet/mellanox/mlx5/core/diag/fw_tracer.c:549:2,
    inlined from 'mlx5_tracer_print_trace' at
drivers/net/ethernet/mellanox/mlx5/core/diag/fw_tracer.c:574:2:
./include/linux/string.h:305:9: warning: '__builtin_strncpy' output may
be truncated copying 256 bytes from a string of length 511
[-Wstringop-truncation]
  return __builtin_strncpy(p, q, size);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix it by using the new strscpy_pad() since the commit 458a3bf82df4
("lib/string: Add strscpy_pad() function") which will always
NUL-terminate the string, and avoid possibly leak data through the ring
buffer where non-admin account might enable these events through perf.

Fixes: fd1483fe1f9f ("net/mlx5: Add support for FW reporter dump")
Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>