dax: add tracepoint to dax_writeback_one()

dax: add tracepoint to dax_writeback_one()

Add a tracepoint to dax_writeback_one(), following the same logging
conventions as the rest of DAX.

Here is an example range writeback which ends up flushing one PMD and
one PTE:

  test-1265  [003] ....
   496.615250: dax_writeback_range: dev 259:0 ino 0x1003 pgoff 0x0-0x7ffffffffffff

  test-1265  [003] ....
   496.616263: dax_writeback_one: dev 259:0 ino 0x1003 pgoff 0x0 pglen 0x200

  test-1265  [003] ....
   496.616270: dax_writeback_one: dev 259:0 ino 0x1003 pgoff 0x305 pglen 0x1

  test-1265  [003] ....
   496.616272: dax_writeback_range_done: dev 259:0 ino 0x1003 pgoff 0x0-0x7ffffffffffff

[[email protected]: struct blk_dax_ctl has disappeared]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ross Zwisler <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Steven Rostedt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>