[XFS] don't block in xfs_qm_dqflush() during async writeback.

Normally dquots are written back via delayed write mechanisms. They are
flushed to their backing buffer by xfssyncd, which is then pushed out by
either AIL or xfsbufd flushing. The flush from the xfssyncd is supposed to
be non-blocking, but xfs_qm_dqflush() always waits for pinned duots, which
means that it will block for the length of time it takes to do a
synchronous log force. This causes unnecessary extra log I/O to be issued
whenever we try to flush a busy dquot.

Avoid the log forces and blocking xfssyncd by making xfs_qm_dqflush() pay
attention to what type of sync it is doing when it sees a pinned dquot and
not waiting when doing non-blocking flushes.

SGI-PV: 988147

SGI-Modid: xfs-linux-melb:xfs-kern:32287a

Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Peter Leckie <pleckie@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>