bcachefs: Add missing restart handling to check_topology()

The next patch will add logging of the specific error being corrected in
repair paths to the journal; this means __bch2_fsck_err() can return
transaction restarts in places that previously weren't expecting them.

check_topology() is old code that doesn't use btree iterators for btree
node locking - it'll have to be rewritten in the future to work online.

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