rust: safety: introduce `unsafe_precondition_assert!` macro

rust: safety: introduce `unsafe_precondition_assert!` macro

Introduce a new `safety` module containing `unsafe_precondition_assert!`
macro. It is a wrapper around `debug_assert!`, intended for validating
preconditions of unsafe function.

When `CONFIG_RUST_DEBUG_ASSERTIONS` flag is enabled, this macro performs
runtime checks to ensure that the preconditions for unsafe function hold.
Otherwise, the macro is a no-op.

Suggested-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1162
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/291566-Library/topic/.60unsafe_precondition_assert.60.20macro/with/528457452
Signed-off-by: Ritvik Gupta <ritvikfoss@gmail.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20251007215034.213779-1-ritvikfoss@gmail.com
[ Added trailing periods, intra-doc link, "a" in "is a no-op" and `()`
  to function reference. Removed plural in assertion message and title
  of macro. Reworded slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>