platform/x86/amd/hsmp: Serialize the data plane against socket teardown

Before this change the HSMP data plane runs without any coordination with
driver teardown: open /dev/hsmp fds and hwmon sysfs reads call
hsmp_send_message() while probe and remove bring sockets up and down.
misc_deregister() does not drain already-open fds, so an in-flight message
can race a concurrent unbind and touch a freed socket array or an unmapped
mailbox.

Add the read side of hsmp_sock_rwsem to the data plane. Split the message
send into hsmp_send_message_locked(), which does the bounds check and MMIO
access and asserts the rwsem is held, and hsmp_send_message(), which wraps
it in guard(rwsem_read). Probe and remove hold the rwsem for write, so they
drain in-flight messages and keep new ones out while they tear a socket
down.

The probe-time senders run under the probe write lock and so must not take
the rwsem again: route hsmp_test(), hsmp_cache_proto_ver() and
hsmp_get_tbl_dram_base() through hsmp_send_message_locked() to avoid
recursive locking. A single rwsem therefore covers both the data plane and
the probe/remove handshake, with no separate probe lock:

 - acpi.c already holds it for write across probe for the socket-array and
   misc-registration handshake, so the mailbox handshake now nests under
   that same lock.

 - plat.c takes it for write around init_platform_device(). It is not held
   across devm_add_action_or_reset() so the release action, which also
   takes it for write, cannot deadlock if that registration fails.

Signed-off-by: Muralidhara M K <muralidhara.mk@amd.com>
Link: https://patch.msgid.link/20260723094656.3806028-7-muralidhara.mk@amd.com
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

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