[XFS] Fix extent list corruption in xfs_iext_irec_compact_full().

If we don't move all the records from the next buffer into the current
buffer then we need to update the er_extoff field of the next buffer as we
shift the remaining records to the start of the buffer.

SGI-PV: 987159

SGI-Modid: xfs-linux-melb:xfs-kern:32165a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Russell Cattelan <cattelan@thebarn.com>