The block reservation mechanism has been broken since the per-cpu
superblock counters were introduced. Make the block reservation code work
with the per-cpu counters by syncing the counters, snapshotting the amount
of available space and then doing a modifcation of the counter state
according to the result. Continue in a loop until we either have no space
available or we reserve some space.

SGI-PV: 956323
SGI-Modid: xfs-linux-melb:xfs-kern:27895a
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NChristoph Hellwig <hch@infradead.org>
Signed-off-by: NTim Shimmin <tes@sgi.com>