[ceph] parse_longname(): strrchr() expects NUL-terminated string

In the Linux kernel, the following vulnerability has been resolved:

[ceph] parse_longname(): strrchr() expects NUL-terminated string

... and parse_longname() is not guaranteed that. That's the reason why it uses kmemdup_nul() to build the argument for kstrtou64(); the problem is, kstrtou64() is not the only thing that need it.

Just get a NUL-terminated copy of the entire thing and be done with that...