btrfs: remove useless logic when finding parent nodes

btrfs: remove useless logic when finding parent nodes

At find_parent_nodes(), at its last step, when iterating over all direct
references, we are checking if we have a share context and if we have
a reference with a different root from the one in the share context.
However that logic is pointless because of two reasons:

1) After the previous patch in the series (subject "btrfs: remove roots
   ulist when checking data extent sharedness"), the roots argument is
   always NULL when using a share check context (struct share_check), so
   this code is never triggered;

2) Even before that previous patch, we could not hit this code because
   if we had a reference with a root different from the one in our share
   context, then we would have exited earlier when doing either of the
   following:

      - Adding a second direct ref to the direct refs red black tree
        resulted in extent_is_shared() returning true when called from
        add_direct_ref() -> add_prelim_ref(), after processing delayed
        references or while processing references in the extent tree;

      - When adding a second reference to the indirect refs red black
        tree (same as above, extent_is_shared() returns true);

      - If we only have one indirect reference and no direct references,
        then when resolving it at resolve_indirect_refs() we immediately
        return that the target extent is shared, therefore never reaching
        that loop that iterates over all direct references at
        find_parent_nodes();

      - If we have 1 indirect reference and 1 direct reference, then we
        also exit early because extent_is_shared() ends up returning true
        when called through add_prelim_ref() (by add_direct_ref() or
        add_indirect_ref()) or add_delayed_refs(). Same applies as when
        having a combination of direct, indirect and indirect with missing
        key references.

   This logic had been obsoleted since commit 3ec4d3238ab165 ("btrfs:
   allow backref search checks for shared extents"), which introduced the
   early exits in case an extent is shared.

So just remove that logic, and assert at find_parent_nodes() that when we
have a share context we don't have a roots ulist and that we haven't found
the extent to be directly shared after processing delayed references and
all references from the extent tree.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>