gve: Use ethtool_sprintf/puts() to fill stats strings

gve: Use ethtool_sprintf/puts() to fill stats strings

Make use of standard helpers to simplify filling in stats strings.

The first two ethtool_puts() changes address the following fortification
warnings flagged by W=1 builds with clang-18. (The last ethtool_puts
change does not because the warning relates to writing beyond the first
element of an array, and gve_gstrings_priv_flags only has one element.)

.../fortify-string.h:562:4: warning: call to '__read_overflow2_field' declared with 'warning' attribute: detected read beyond size of field (2nd parameter); maybe use struct_group()? [-Wattribute-warning]
  562 |                         __read_overflow2_field(q_size_field, size);
      |                         ^
.../fortify-string.h:562:4: warning: call to '__read_overflow2_field' declared with 'warning' attribute: detected read beyond size of field (2nd parameter); maybe use struct_group()? [-Wattribute-warning]

Likewise, the same changes resolve the same problems flagged by Smatch.

.../gve_ethtool.c:100 gve_get_strings() error: __builtin_memcpy() '*gve_gstrings_main_stats' too small (32 vs 576)
.../gve_ethtool.c:120 gve_get_strings() error: __builtin_memcpy() '*gve_gstrings_adminq_stats' too small (32 vs 512)

Compile tested only.

Reviewed-by: Shailend Chand <shailend@google.com>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Signed-off-by: Simon Horman <horms@kernel.org>
Acked-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20240508-gve-comma-v2-2-1ac919225f13@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>