bcachefs: btree read retry fixes

Fix btree node read retries after validate errors:

__btree_err() is the wrong place to flag a topology error: that is done
by btree_lost_data().

Additionally, some calls to bch2_bkey_pick_read_device() were not
updated in the 6.16 rework for improved log messages; we were failing to
signal that we still had a retry.

Cc: Nikita Ofitserov <himikof@gmail.com>
Cc: Alan Huang <mmpgouride@gmail.com>
Reported-and-tested-by: Edoardo Codeglia <bcachefs@404.blue>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>