mds: make FSMap.get_filesystem return const

mds: make FSMap.get_filesystem return const

To make it clearer that anyone modifying a filesystem
should be using modify_filesystem()

Note that FSMap is still a friend of MDSMonitor, as
MDSMap was, so a larger refactor to provide the appropriate
const accessors would also be nice in the future, but that's
nothign to do with multi-filesystems.

Signed-off-by: John Spray <john.spray@redhat.com>