copy: drop COPY_REFLINK

Reflinking is a safe optimization whenever the source and destination are
regular, seekable files on a filesystem that supports cloning. Requiring each
caller to opt in through COPY_REFLINK adds flag plumbing without changing the
necessary fallback behavior.

Drop COPY_REFLINK, renumber the remaining flags, and have copy_bytes_full()
unconditionally try FICLONE or FICLONERANGE before entering its normal copy
loop. Isolate the clone attempt and its file-offset bookkeeping in a helper so
copy_bytes_full() only has to handle its cloned, unavailable, and error results.
A successful clone therefore completes in one operation, while an unsupported
or rejected clone falls back to progress-limited copy_file_range(), sendfile(),
or buffered copying. Keep the public reflink helpers for operations that
specifically require clone semantics.

Existing test-copy coverage exercises both bounded and unbounded regular-file
copies through copy_bytes_full().

Signed-off-by: Daan De Meyer <daan@amutable.com>

src/bootctl/bootctl-install.c

@@ -523,7 +523,7 @@ static int copy_file_with_version_check(
          * might be left at the end of the file. (Resetting before rather than after a copy attempt is safer
          * because a previous attempt might have failed half-way, leaving the file offset at some undefined
          * place.) */
-        r = copy_bytes(source_fd, write_fd, UINT64_MAX, COPY_REFLINK|COPY_SEEK0_SOURCE);
+        r = copy_bytes(source_fd, write_fd, UINT64_MAX, COPY_SEEK0_SOURCE);
         if (r < 0)
                 return log_error_errno(r, "Failed to copy data from \"%s\" to \"%s\": %m", source_path, dest_path);
 

src/bootctl/bootctl-link.c

@@ -453,7 +453,7 @@ static int begin_copy_file(
         CLEANUP_TMPFILE_AT(target_dir_fd, t);
 
         if (source_fd >= 0) {
-                r = copy_bytes(source_fd, write_fd, UINT64_MAX, COPY_REFLINK|COPY_SEEK0_SOURCE);
+                r = copy_bytes(source_fd, write_fd, UINT64_MAX, COPY_SEEK0_SOURCE);
                 if (r < 0)
                         return log_error_errno(r, "Failed to copy data into '%s': %m", filename);