vfs_btrfs does unwanted read/write copy fallback for FSCTL_DUP_EXTENTS_TO_FILE

It seems the behaviour to fail the FSCTL_DUP_EXTENTS_TO_FILE if the ioctl() failed was never really implemented.

Commit 373d3015 probably should have checked VFS_COPY_CHUNK_FL_MUST_CLONE in vfs_btrfs.c, but it seems this was overlooked.

After changing the VFS API from copy-chunk to offload read/write the check is still missing from vfs_default offload-write implementation.