bcachefs: Convert for_each_btree_node() to lockrestart_do()

for_each_btree_node() now works similarly to for_each_btree_key(), where
the loop body is passed as an argument to be passed to lockrestart_do().

This now calls trans_begin() on every loop iteration - which fixes an
SRCU warning in backpointers fsck.

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