xfs: wait for I/O completion when writing out pages in xfs_setattr_size

The current code relies on the xfs_ioend_wait call later on to make sure
all I/O actually has completed.  The xfs_ioend_wait call will go away soon,
so prepare for that by using the waiting filemap function.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>