bcachefs: support REMAP_FILE_DEDUP in bch2_remap_file_range

By removing the early-exit when REMAP_FILE_DEDUP is set, we should be
able to support the fideduperange ioctl, albeit less efficiently than if
we handled some of the extent locking and comparison logic inside
bcachefs.  Extent comparison logic already exists inside of
`__generic_remap_file_range_prep`.

Signed-off-by: Reed Riley <reed@riley.engineer>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>