install: Don't skip syncing in the common case.

[des]

Summary

In copy(), if no digest was requested (which is the common case), we
use copy_file_range() to avoid needlessly copying the contents of the
file into user space and back. When copy_file_range() returns
successfully (which, again, is the common case), we simply return, and
therefore never get to the point where we call fsync() if the -S
option was specified. Fix this.

MFC after: 1 week
Sponsored by: Klara, Inc.

[markj]

Thanks!