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/[email protected]
Signed-off-by: Brian Foster <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Cc: Dave Chinner <[email protected]>
cc: Josef Bacik <[email protected]>
Cc: Holger Hoffstätte <[email protected]>
Cc: Al Viro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>