libbpf: Use strscpy() in kernel code for skel_map_create()

libbpf: Use strscpy() in kernel code for skel_map_create()

Linux has deprecated[1] strncpy(), and the use in skel_map_create()
is best replaced with strscpy(). Since we still need to build this
file in userspace, leave the strncpy() in place in that case. This
is the last use of strncpy() in the kernel.

Link: https://github.com/KSPP/linux/issues/90
Signed-off-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20260513050806.do.620-kees@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>