rpmsg: glink: Replace strncpy() with strscpy_pad()

rpmsg: glink: Replace strncpy() with strscpy_pad()

The use of strncpy() is considered deprecated for NUL-terminated
strings[1]. Replace strncpy() with strscpy_pad() (as it seems this case
expects the NUL padding to fill the allocation following the flexible
array). This additionally silences a warning seen when building under
-Warray-bounds:

./include/linux/fortify-string.h:38:30: warning: '__builtin_strncpy' offset 24 from the object at '__mptr' is out of the bounds of referenced subobject 'data' with type 'u8[]' {aka 'unsigned char[]'} at offset 24 [-Warray-bounds]
   38 | #define __underlying_strncpy __builtin_strncpy
      |                              ^
./include/linux/fortify-string.h:50:9: note: in expansion of macro '__underlying_strncpy'
   50 |  return __underlying_strncpy(p, q, size);
      |         ^~~~~~~~~~~~~~~~~~~~
drivers/rpmsg/qcom_glink_native.c: In function 'qcom_glink_work':
drivers/rpmsg/qcom_glink_native.c:36:5: note: subobject 'data' declared here
   36 |  u8 data[];
      |     ^~~~

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings

Cc: Andy Gross <agross@kernel.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: linux-remoteproc@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/lkml/20210728020745.GB35706@embeddedor
Link: https://lore.kernel.org/r/20210818060533.3569517-4-keescook@chromium.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>