tools: ynl: fix string attribute length to include null terminator

The ynl_attr_put_str() function was not including the null terminator
in the attribute length calculation. This caused kernel to reject
CTRL_CMD_GETFAMILY requests with EINVAL:
"Attribute failed policy validation".

For a 4-character family name like "dpll":
- Sent: nla_len=8 (4 byte header + 4 byte string without null)
- Expected: nla_len=9 (4 byte header + 5 byte string with null)

The bug was introduced in commit 15d2540e0d62 ("tools: ynl: check for
overflow of constructed messages") when refactoring from stpcpy() to
strlen(). The original code correctly included the null terminator:

end = stpcpy(ynl_attr_data(attr), str);
attr->nla_len = NLA_HDRLEN + NLA_ALIGN(end -
(char *)ynl_attr_data(attr));

Since stpcpy() returns a pointer past the null terminator, the length
included it. The refactored version using strlen() omitted the +1.

The fix also removes NLA_ALIGN() from nla_len calculation, since
nla_len should contain actual attribute length, not aligned length.
Alignment is only for calculating next attribute position. This makes
the code consistent with ynl_attr_put().

CTRL_ATTR_FAMILY_NAME uses NLA_NUL_STRING policy which requires
null terminator. Kernel validates with memchr() and rejects if not
found.

Fixes: 15d2540e0d62 ("tools: ynl: check for overflow of constructed messages")
Signed-off-by: Petr Oros <[e-mail]>
Tested-by: Ivan Vecera <[e-mail]>
Reviewed-by: Ivan Vecera <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/net/ynl/lib/ynl-priv.h

···
 	struct nlattr *attr;
 	size_t len;
 
-	len = strlen(str);
+	len = strlen(str) + 1;
 	if (__ynl_attr_put_overflow(nlh, len))
 		return;
 
···
 	attr->nla_type = attr_type;
 
 	strcpy((char *)ynl_attr_data(attr), str);
-	attr->nla_len = NLA_HDRLEN + NLA_ALIGN(len);
+	attr->nla_len = NLA_HDRLEN + len;
 
 	nlh->nlmsg_len += NLMSG_ALIGN(attr->nla_len);
 }