gallium/u_transfer_helper: Free the staging buffer on unmap.

u_transfer_helper sometimes mallocs a staging buffer, and leaked it.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
10 jobs for !1049 with transfer-leak in 8 minutes and 40 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Containers Build
passed #348088
debian

00:00:17

 
  Build+Test
passed #348090
meson-clang

00:06:18

passed #348094
meson-clover

00:08:22

passed #348093
meson-main

00:05:39

passed #348089
meson-swr-glvnd

00:05:53

passed #348095
meson-vulkan

00:02:32

passed #348097
scons-llvm

00:03:33

passed #348096
scons-nollvm

00:03:28

passed #348091
scons-swr

00:05:58

passed #348092
scons-win64

00:06:33