Chimera: fix out-by-one error

The limit on a filename is 255 characters.  Due to an out-by-one error, for
exactly 255 character-length filenames the JdbFs layer fails to catch the
problem and the database catches it instead.

Target: master
Requires-book: no
Requires-notes: no
Patch: http://rb.dcache.org/r/6183/
Acked-by: Tigran Mkrtchyan