bcachefs: Fix extents iteration + snapshots interaction

bcachefs: Fix extents iteration + snapshots interaction

peek_upto() checks against the end position and bails out before
FILTER_SNAPSHOTS checks; this is because if we end up at a different
inode number than the original search key none of the keys we see might
be visibile in the current snapshot - we might be looking at inode in a
completely different subvolume.

But this is broken, because when we're iterating over extents we're
checking against the extent start position to decide when to bail out,
and the extent start position isn't monotonically increasing until after
we've run FILTER_SNAPSHOTS.

Fix this by adding a simple inode number check where the old bailout
check was, and moving the main check to the correct position.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Reported-by: "Carl E. Thompson" <list-bcachefs@carlthompson.net>