[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...

Fixes: dd66df00 "ceph: add support for encrypted snapshot names"
Tested-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>