kallsyms: Use bounded strnchr() when parsing string

When parsing for the <module:name> format, we use strchr() to look for
the separator, when we know that the module name can't be longer than
MODULE_NAME_LEN. Enforce the same using strnchr().

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Jessica Yu <jeyu@redhat.com>