calculate index in the FsExport constructor

While debugging slow NFS performance accompanied by high CPU usage noticed that FsExport.getIndex() determines some index on every call from what seems like immutable data. Moved it to constructor.

Target: master
Request: 0.7
Acked-by: Paul Millar <paul.millar@desy.de> Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7372/
(cherry picked from commit 1ee6f7a2b91725ddee7801d5383b86039829b208)