Both copy_file_range() and read() / write() in our fallback loop can be
interrupted before copying anything at all, in which case it returns -1
and sets errno to EINTR. If that happens, we should retry, not fail.

While here, drop the size argument from copy() (we always want to copy
the entire file anyway) and add a test case that exercises the metalog
and digest functionality.

MFC after: 1 week
PR: 293028