mv: no warning when the destination file system doesn't support xattrs (GNU cp-mv-enotsup-xattr, 9.12) #15079

Since the bump to GNU 9.12 (#14564), tests/cp/cp-mv-enotsup-xattr fails on the mv half. When a cross-device move lands on a file system without xattr support, GNU now prints

mv: setting attributes for 'noxattr/a': Operation not supported

and still exits 0. We print nothing.

The 9.12 manual for mv says: "Upon failure all warnings are emitted, but do not modify the exit status." Older manuals said all warnings except "Operation not supported", which is what #10083 implemented for 9.9. The fd-based copy in rename_file_fallback still calls copy_xattrs_fd_ignore_unsupported, so the ENOTSUP case stays silent.

cp's half of the test passes, so this is mv only. I'll send a PR for the file path. The directory paths are being reworked in #14971, so I'll leave those alone.