bcachefs: Fix btree iter flags in data move (2)

bcachefs: Fix btree iter flags in data move (2)

Data move -> move_get_io_opts -> bch2_get_update_rebalance_opts

requires a not_extents iterator; this fixes the path where we're walking
the extents btree and chase a reflink pointer into the reflink btree.

bch2_lookup_indirect_extent() requires working with an extents iterator
(due to peek_slot() semantics), so we implement
bch2_lookup_indirect_extent_for_move().

This is simplified because there's no need to report
indirect_extent_missing_errors here, that can be deferred until fsck or
when a user reads that data.

Reported-by: Maël Kerbiriou <mael.kerbiriou@free.fr>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>