copy_file_range.2: Update cross-filesystem support for 5.12

copy_file_range.2: Update cross-filesystem support for 5.12

Linux 5.12 fixes a regression.

Cross-filesystem (introduced in 5.3) copies were buggy.

Move the statements documenting cross-fs to BUGS.
Kernels 5.3..5.11 should be patched soon.

State version information for some errors related to this.

Reported-by: Luis Henriques <lhenriques@suse.de>
Reported-by: Amir Goldstein <amir73il@gmail.com>
Related: <https://lwn.net/Articles/846403/>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Anna Schumaker <anna.schumaker@netapp.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Steve French <sfrench@samba.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Nicolas Boichat <drinkcat@chromium.org>
Cc: Ian Lance Taylor <iant@google.com>
Cc: Luis Lozano <llozano@chromium.org>
Cc: Andreas Dilger <adilger@dilger.ca>
Cc: Olga Kornievskaia <aglo@umich.edu>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: ceph-devel <ceph-devel@vger.kernel.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>
Cc: CIFS <linux-cifs@vger.kernel.org>
Cc: samba-technical <samba-technical@lists.samba.org>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>
Cc: Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Cc: Walter Harms <wharms@bfs.de>
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>