exportfs: Change bcachefs fid_type enum to avoid conflicts

Per Amir Goldstein, the fid types that bcachefs picked conflicted with
xfs and fuse, which previously were in use but not deviced in the master
enum.

Since bcachefs is still out of tree, we can move.

https://lore.kernel.org/linux-next/20231026203733.fx65mjyic4pka3e5@moria.home.lan/T/#ma59f65ba61f605b593e69f4690dbd317526d83ba



Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>