xfs: fix a use after free in xfs_end_io_direct_write

There is a window in which the ioend that we call inode_dio_wake on
in xfs_end_io_direct_write is already free.  Fix this by storing
the inode pointer in a local variable.

This is a fix for the regression introduced in 3.1-rc by
"fs: move inode_dio_done to the end_io handler".

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