xfs: enable buffer deadlock postmortem diagnosis via ftrace

Create a second buf_trylock tracepoint so that we can distinguish
between a successful and a failed trylock.  With this piece, we can
use a script to look at the ftrace output to detect buffer deadlocks.

[dchinner: update to if/else as per hch's suggestion]

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>