[XFS] Fix a synchronous buftarg flush deadlock when freezing.

[XFS] Fix a synchronous buftarg flush deadlock when freezing.

At the last stage of a freeze, we flush the buftarg synchronously over and
over again until it succeeds twice without skipping any buffers.

The delwri list flush skips pinned buffers, but tries to flush all others.
It removes the buffers from the delwri list, then tries to lock them one
at a time as it traverses the list to issue the I/O. It holds them locked
until we issue all of the I/O and then unlocks them once we've waited for
it to complete.

The problem is that during a freeze, the filesystem may still be doing
stuff - like flushing delalloc data buffers - in the background and hence
we can be trying to lock buffers that were on the delwri list at the same
time. Hence we can get ABBA deadlocks between threads doing allocation and
the buftarg flush (freeze) thread.

Fix it by skipping locked (and pinned) buffers as we traverse the delwri
buffer list.

SGI-PV: 957195
SGI-Modid: xfs-linux-melb:xfs-kern:27535a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>