bcachefs: bcachefs_metadata_version_reflink_p_may_update_opts

bcachefs: bcachefs_metadata_version_reflink_p_may_update_opts

Previously, io path option changes on a file would be picked up
automatically and applied to existing data - but not for reflinked data,
as we had no way of doing this safely. A user may have had permission to
copy (and reflink) a given file, but not write to it, and if so they
shouldn't be allowed to change e.g. nr_replicas or other options.

This uses the incompat feature mechanism in the previous patch to add a
new incompatible flag to bch_reflink_p, indicating whether a given
reflink pointer may propagate io path option changes back to the
indirect extent.

In this initial patch we're only setting it for the source extents.

We'd like to set it for the destination in a reflink copy, when the user
has write access to the source, but that requires mnt_idmap which is not
curretly plumbed up to remap_file_range.

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