When processing multiple extent maps, xfs_bmapi needs to keep track of the
extent behind the one it is currently working on to be able to trim extent
ranges correctly. Failing to update the previous pointer can result in
corrupted extent lists in memory and this will result in panics or assert
failures.

Update the previous pointer correctly when we move to the next extent to
process.

SGI-PV: 965631
SGI-Modid: xfs-linux-melb:xfs-kern:28773a
Signed-off-by: NDavid Chinner <dgc@sgi.com>
Signed-off-by: NVlad Apostolov <vapo@sgi.com>
Signed-off-by: NTim Shimmin <tes@sgi.com>