IB/mlx4: Replace kfree with kvfree in mlx4_ib_destroy_srq

Commit 0ef2f05c uses vmalloc for WR buffers
when needed and uses kvfree to free the buffers. It missed changing kfree
to kvfree in mlx4_ib_destroy_srq().

Reported-by: Matthew Finaly <matt@Mellanox.com>
Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>