chimera: update tags mtime on re-write

keep nfs client cache happy

Acked-by: Dmitry Litvintsev
Patch: http://rb.dcache.org/r/5617/
Target: trunk, 2.6, 2.2
Require-book: no
Require-notes: no
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>