firmware: arm_ffa: Fix out-of-bound writes in ffa_setup_and_transmit()

firmware: arm_ffa: Fix out-of-bound writes in ffa_setup_and_transmit()

Sashiko (locally) reports multiple out-of-bound issues in
ffa_setup_and_transmit:
1) Writing ep_mem_access->reserved can write out of bounds for FFA
   versions < 1.2 as ffa_emad_size_get() returns 16 bytes in that case
   while reserved has an offset of 24.
   Instead of zeroing fields, memset the struct to zero first based on
   the FFA version.

2) Make sure there is enough size to write constituents.

While at it, convert the only sizeof() in the driver that uses a
type instead of variable.

Reviewed-by: Sudeep Holla <sudeep.holla@kernel.org>
Fixes: 111a833dc5cb ("firmware: arm_ffa: Set reserved/MBZ fields to zero in the memory descriptors")
Signed-off-by: Mostafa Saleh <smostafa@google.com>
Signed-off-by: Sebastian Ene <sebastianene@google.com>
Link: https://patch.msgid.link/20260702103848.1647249-2-sebastianene@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>