rust: miscdevice: add missing safety comments

This fixes the following four warnings:

	warning: unsafe block missing a safety comment
	   --> /home/aliceryhl/rust-for-linux/rust/kernel/miscdevice.rs:168:15
	    |
	168 |             ..unsafe { MaybeUninit::zeroed().assume_init() }
	    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	    |
	    = help: consider adding a safety comment on the preceding line
	    = help: for further information visit
	      https://rust-lang.github.io/rust-clippy/master/index.html#undocumented_unsafe_blocks
	    = note: requested on the command line with `-W clippy::undocumented-unsafe-blocks`

	warning: unsafe function's docs are missing a `# Safety` section
	   --> /home/aliceryhl/rust-for-linux/rust/kernel/miscdevice.rs:175:1
	    |
	175 | / unsafe extern "C" fn fops_open<T: MiscDevice>(
	176 | |     inode: *mut bindings::inode,
	177 | |     file: *mut bindings::file,
	178 | | ) -> c_int {
	    | |__________^
	    |
	    = help: for further information visit
	      https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc
	    = note: `-W clippy::missing-safety-doc` implied by `-W clippy::all`
	    = help: to override `-W clippy::all` add `#[allow(clippy::missing_safety_doc)]`

	warning: unsafe function's docs are missing a `# Safety` section
	   --> /home/aliceryhl/rust-for-linux/rust/kernel/miscdevice.rs:196:1
	    |
	196 | / unsafe extern "C" fn fops_release<T: MiscDevice>(
	197 | |     _inode: *mut bindings::inode,
	198 | |     file: *mut bindings::file,
	199 | | ) -> c_int {
	    | |__________^
	    |
	    = help: for further information visit
	      https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc

	warning: unsafe function's docs are missing a `# Safety` section
	   --> /home/aliceryhl/rust-for-linux/rust/kernel/miscdevice.rs:210:1
	    |
	210 | / unsafe extern "C" fn fops_ioctl<T: MiscDevice>(
	211 | |     file: *mut bindings::file,
	212 | |     cmd: c_uint,
	213 | |     arg: c_ulong,
	214 | | ) -> c_long {
	    | |___________^
	    |
	    = help: for further information visit
	      https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc



Note that these warnings are currently not enabled in the build, but
rust-next contains a commit that will enable them, so we should fix
them.

Reported-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Closes: https://lore.kernel.org/rust-for-linux/CANiq72kOs6vPDUzZttQNqePFHphCQ30iVmZ5MO7eCJfPG==Vzg@mail.gmail.com/


Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://lore.kernel.org/r/20241022-miscdevice-unsafe-warn-fix-v1-1-a78fde1740d6@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>