bcachefs: btree_and_journal_iter: don't iterate over too many whiteouts when prefetching

bcachefs: btree_and_journal_iter: don't iterate over too many whiteouts when prefetching

To help ameloriate issues with peek operations having to skip over
deletions in the journal - just bail out if all we're doing is
prefetching btree nodes.

Since btree node prefetching runs every time we iterate to a new node,
and has to sequentially scan ahead, this avoids another O(n^2).

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