kallsyms: Handle too long symbols in kallsyms.c

mainline inclusion
from mainline-5.0-rc8
commit 6db2983c
category: bugfix
bugzilla: 10868
CVE: NA

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When checking for symbols with excessively long names,
account for null terminating character.

Fixes: f3462aa9 ("Kbuild: Handle longer symbols in kallsyms.c")
Signed-off-by: Eugene Loh <eugene.loh@oracle.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Reviewed-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>