optee: immediately free RPC buffers that are released by OP-TEE

optee: immediately free RPC buffers that are released by OP-TEE

This commit fixes a case overlooked in [1].

There are two kinds of shared memory buffers used by OP-TEE:
1. Normal payload buffer
2. Internal command structure buffers

The internal command structure buffers are represented with a shadow
copy internally in Xen since this buffer can contain physical addresses
that may need to be translated between real physical address and guest
physical address without leaking information to the guest.

[1] fixes the problem when releasing the normal payload buffers. The
internal command structure buffers must be released in the same way.
Failure to follow this order opens a window where the guest has freed
the shared memory but Xen is still tracking the buffer.

During this window the guest may happen to recycle this particular
shared memory in some other thread and try to use it. Xen will block
this which will lead to spurious failures to register a new shared
memory block.

Fix this by freeing the internal command structure buffers first before
informing the guest that the buffer can be freed.

[1] 5b13eb1d978e ("optee: immediately free buffers that are released by OP-TEE")

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
[stefano: minor code style fix]
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>