xfs: remove if_lastex

The if_lastex field in struct xfs_ifork is only used as a temporary
index during xfs_bmapi and xfs_bunmapi.  Instead of using the inode
fork to store it keep it local in the callchain.  Fortunately this
is very easy as we already pass a stack copy of it down the whole
chain which can simplify be changed to be passed by reference.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>