cxgb4: Use kfree() instead kvfree() where appropriate

Use kfree(buf) in blocked_fl_read() because the memory is allocated with
kzalloc(). Use kfree(t) in blocked_fl_write() because the memory is
allocated with kcalloc().

Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>