vfs: allow copy_file_range to copy across devices

We want to enable cross-filesystem copy_file_range functionality
where possible, so push the "same superblock only" checks down to
the individual filesystem callouts so they can make their own
decisions about cross-superblock copy offload and fallack to
generic_copy_file_range() for cross-superblock copy.

[Amir] We do not call ->remap_file_range() in case the files are not
on the same sb and do not call ->copy_file_range() in case the files
do not belong to the same filesystem driver.

This changes behavior of the copy_file_range(2) syscall, which will
now allow cross filesystem in-kernel copy.  CIFS already supports
cross-superblock copy, between two shares to the same server. This
functionality will now be available via the copy_file_range(2) syscall.

Cc: Steve French <stfrench@microsoft.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/nfs/nfs4file.c

@@ -133,6 +133,9 @@ static ssize_t __nfs4_copy_file_range(struct file *file_in, loff_t pos_in,
 				      struct file *file_out, loff_t pos_out,
 				      size_t count, unsigned int flags)
 {
+	/* Only offload copy if superblock is the same */
+	if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
+		return -EXDEV;
 	if (!nfs_server_capable(file_inode(file_out), NFS_CAP_COPY))
 		return -EOPNOTSUPP;
 	if (file_inode(file_in) == file_inode(file_out))
@@ -148,7 +151,7 @@ static ssize_t nfs4_copy_file_range(struct file *file_in, loff_t pos_in,
 
 	ret = __nfs4_copy_file_range(file_in, pos_in, file_out, pos_out, count,
 				     flags);
-	if (ret == -EOPNOTSUPP)
+	if (ret == -EOPNOTSUPP || ret == -EXDEV)
 		ret = generic_copy_file_range(file_in, pos_in, file_out,
 					      pos_out, count, flags);
 	return ret;