fs/fs-writeback.c: inode writeback list tracking tracepoints

fs/fs-writeback.c: inode writeback list tracking tracepoints

The per-sb inode writeback list tracks inodes currently under writeback
to facilitate efficient sync processing.  In particular, it ensures that
sync only needs to walk through a list of inodes that were cleaned by
the sync.

Add a couple tracepoints to help identify when inodes are added/removed
to and from the writeback lists.  Piggyback off of the writeback
lazytime tracepoint template as it already tracks the relevant inode
information.

Link: http://lkml.kernel.org/r/1466594593-6757-3-git-send-email-bfoster@redhat.com
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Dave Chinner <dchinner@redhat.com>
cc: Josef Bacik <jbacik@fb.com>
Cc: Holger Hoffstätte <holger.hoffstaette@applied-asynchrony.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>