UPSTREAM: mailbox: mailbox-test: fix a locking issue in mbox_test_message_write()

[ Upstream commit 8fe72b76db79d694858e872370df49676bc3be8c ]

There was a bug where this code forgot to unlock the tdev->mutex if the
kzalloc() failed.  Fix this issue, by moving the allocation outside the
lock.

Bug: 275340532
Fixes: 2d1e952a2b8e ("mailbox: mailbox-test: Fix potential double-free in mbox_test_message_write()")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 7d233f93594f0d9afe44e9409131a8d6ad4f593c)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I7a4a1bf06abbb2092aceb72610e3f894b2bfbf0f