chimera: special files mtime should be always current time

recently added '.(get)(<file>)(locality)' special file uses mtime from the original file.
As file is not changes the mtime get not updated. This tells NFS client that content is
not changes and cached value is OK.

Set mtime always to be current time to invalidate NFS client cache.

Acked-by: Paul Millar
Target: trunk, 2.6
Require-book: no
Require-notes: no
patch: http://rb.dcache.org/r/5471/
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>