Like all filesystem manipulation tools,
xfs_fsr takes care of leaving the filesystem in a consistent state, in case the machine crashes (due to a power failure, for example). Unless you're unlucky and encounter a bug, that is — filesystem drivers are more complex than they look, especially as they are written for speed.
If you interrupt
xfs_fsr cleanly (with any of the usual signals SIGINT, SIGHUP, SIGTERM or SIGQUIT), it takes care to write where it left off in
/var/tmp/.fsrlast (or the file indicated with the
-f option). So you can safely interrupt it with Ctrl+C, and restart it again with the same options later to complete the job.