rust: miscdevice: access the `struct miscdevice` from fops->open()

rust: miscdevice: access the `struct miscdevice` from fops->open()

Providing access to the underlying `struct miscdevice` is useful for
various reasons. For example, this allows you access the miscdevice's
internal `struct device` for use with the `dev_*` printing macros.

Note that since the underlying `struct miscdevice` could get freed at
any point after the fops->open() call (if misc_deregister is called),
only the open call is given access to it. To use `dev_*` printing macros
from other fops hooks, take a refcount on `miscdevice->this_device` to
keep it alive. See the linked thread for further discussion on the
lifetime of `struct miscdevice`.

Link: https://lore.kernel.org/r/2024120951-botanist-exhale-4845@gregkh
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Lee Jones <lee@kernel.org>
Tested-by: Lee Jones <lee@kernel.org>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241210-miscdevice-file-param-v3-2-b2a79b666dc5@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>