Btrfs: add work_struct information for workqueue tracepoint

Kernel workqueue's tracepoints print the address of work_struct, while btrfs
workqueue's tracepoints print the address of btrfs_work.

We need a connection between this two, for example when debuging, we usually
grep an address in the trace output.  So it'd be better to also print
work_struct in btrfs workqueue's tracepoint.

Please note that we can only add this into those tracepoints whose work is still
available in memory because we need to reference the work.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>