bcachefs: Fix topology errors on split after merge

If a btree split picks a pivot that's being deleted by a btree node
merge, we're going to have problems.

Fix this by checking if the pivot is being deleted, the same as we check
for deletions in journal replay keys.

Found by single_devic.ktest small_nodes.

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