Fix handling of copy_file_range() return value

Fix handling of copy_file_range() return value

Treat copy_file_range() return value of zero as an error: it indicates
that no bytes could be copied (perhaps the source file is shorter than
expected), and the existing retry loop would otherwise spin forever
since nwritten would never reach BLCKSZ.

The other uses of copy_file_range() in the tree don't have this
problem.

Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Reviewed-by: Yingying Chen <cyy9255@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/3208cf7a-c7f3-41eb-92f6-33cbeff4df40%40eisentraut.org