writeback: fix false warning in inode_to_wb()

inode_to_wb() is used also for filesystems that don't support cgroup
writeback.  For these filesystems inode->i_wb is stable during the
lifetime of the inode (it points to bdi->wb) and there's no need to hold
locks protecting the inode->i_wb dereference.  Improve the warning in
inode_to_wb() to not trigger for these filesystems.

Link: https://lkml.kernel.org/r/20250412163914.3773459-3-agruenba@redhat.com


Fixes: aaa2cacf ("writeback: add lockdep annotation to inode_to_wb()")
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>