test_hexdump: avoid string truncation warning

gcc can warn when a string is too long to fit into the strncpy()
destination buffer, as it is here depending on the function arguments:

inlined from 'test_hexdump_prepare_test.constprop' at /home/arnd/arm-soc/lib/test_hexdump.c:116:3:
include/linux/fortify-string.h:108:33: error: '__builtin_strncpy' output truncated copying between 0 and 32 bytes from a string of length 32 [-Werror=stringop-truncation]
108 | #define __underlying_strncpy __builtin_strncpy
| ^
include/linux/fortify-string.h:187:16: note: in expansion of macro '__underlying_strncpy'
187 | return __underlying_strncpy(p, q, size);
| ^~~~~~~~~~~~~~~~~~~~

The intention here is to copy exactly 'l' bytes without any padding or
NUL-termination, so the most logical change is to use memcpy(), just as
a previous change adapted the other output from strncpy() to memcpy().

Link: https://lkml.kernel.org/r/20240409140059.3806717-2-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Justin Stitt <justinstitt@google.com>
Cc: Alexey Starikovskiy <astarikovskiy@suse.de>
Cc: Bob Moore <robert.moore@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Len Brown <lenb@kernel.org>
Cc: Lin Ming <ming.m.lin@intel.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Nicolas Schier <nicolas@fjasle.eu>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: "Richard Russon (FlatCap)" <ldm@flatcap.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>