xfs: fix freeing memory in xfs_getbmap()

Regression from commit 28e211700a81b0a934b6c7a4b8e7dda843634d2f.
Need to free temporary buffer allocated in xfs_getbmap().

Signed-off-by: Felix Blyakher <felixb@sgi.com>
Signed-off-by: Hedi Berriche <hedi@sgi.com>
Reported-by: Justin Piszcz <jpiszcz@lucidpixels.com>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>