greybus: raw: fix use-after-free if write is called after disconnect

greybus: raw: fix use-after-free if write is called after disconnect

If a user writes to the chardev after disconnect has been called, the
kernel panics with the following trace (with
CONFIG_INIT_ON_FREE_DEFAULT_ON=y):

        BUG: kernel NULL pointer dereference, address: 0000000000000218
         ...
        Call Trace:
         <TASK>
         gb_operation_create_common+0x61/0x180
         gb_operation_create_flags+0x28/0xa0
         gb_operation_sync_timeout+0x6f/0x100
         raw_write+0x7b/0xc7 [gb_raw]
         vfs_write+0xcf/0x420
         ? task_mm_cid_work+0x136/0x220
         ksys_write+0x63/0xe0
         do_syscall_64+0xa4/0x290
         entry_SYSCALL_64_after_hwframe+0x77/0x7f

Disconnect calls gb_connection_destroy, which ends up freeing the
connection object. When gb_operation_sync is called in the write file
operations, its gets a freed connection as parameter and the kernel
panics.

The gb_connection_destroy cannot be moved out of the disconnect
function, as the Greybus subsystem expect all connections belonging to a
bundle to be destroyed when disconnect returns.

To prevent this bug, use a rw lock to synchronize access between write
and disconnect. This guarantees that the write function doesn't try
to use a disconnected connection.

Fixes: e806c7fb8e9b ("greybus: raw: add raw greybus kernel driver")
Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Damien Riégel <damien.riegel@silabs.com>
Link: https://patch.msgid.link/20260324140039.40001-2-damien.riegel@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>