bcachefs: Split out SPOS_MAX

bcachefs: Split out SPOS_MAX

Internal btree code really wants a POS_MAX with all fields ~0; external
code more likely wants the snapshot field to be 0, because when we're
passing it to bch2_trans_get_iter() it's used for the snapshot we're
operating in, which should be 0 for most btrees that don't use
snapshots.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>