chimera: fix sql exception if file name too long

in some cases (at least during testing) we see names longer that
256 symbols. This results into naasty exceptions which is fixed
by this patch.

Patch: http://rb.dcache.org/r/5490
Target: trunk
Request: 2.6
Require-book: no
Require-notes: no
Merge pull request #262 from paulmillar/fix/2.6/long-filename

chimera: fix sql exception if file name too long