btrfs: remove checks for a root with id 0 during backref walking

When doing backref walking to determine if an extent is shared, we are
testing the root_objectid of the given share_check struct is 0, but that
is an impossible case, since btrfs_is_data_extent_shared() always
initializes the root_objectid field with the id of the given root, and
no root can have an objectid of 0. So remove those checks.

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