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>