bcachefs: fix transaction restart handling in check_extents(), check_dirents()

Dealing with outside state within a btree transaction is always tricky.

check_extents() and check_dirents() have to accumulate counters for
i_sectors and i_nlink (for subdirectories). There were two bugs:

- transaction commit may return a restart; therefore we have to commit
  before accumulating to those counters
- get_inode_all_snapshots() may return a transaction restart, before
  updating w->last_pos; then, on the restart,
  check_i_sectors()/check_subdir_count() would see inodes that were not
  for w->last_pos

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