xfs: use kvfree for buf in xfs_ioc_getbmap

Without this the kernel crashes in kfree for files with a sufficiently
large number of extents.

Fixes: d4c75a1b ("xfs: convert remaining kmem_free() to kfree()")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>

fs/xfs/xfs_ioctl.c

@@ -1506,7 +1506,7 @@ xfs_ioc_getbmap(
 
 	error = 0;
 out_free_buf:
-	kfree(buf);
+	kvfree(buf);
 	return error;
 }