rust: num: bounded: Always inline fits_within and from_expr

`from_expr` relies on `build_assert` to infer that the passed expression
fits the type's boundaries at build time. That inference can only be
successful its code (and that of `fits_within`, which performs the
check) is inlined, as a dedicated function would need to work with a
variable and cannot verify that property.

While inlining happens as expected in most cases, it is not guaranteed.
In particular, kernel options that optimize for size like
`CONFIG_CC_OPTIMIZE_FOR_SIZE` can result in `from_expr` not being
inlined.

Add `#[inline(always)]` attributes to both `fits_within` and `from_expr`
to make the compiler inline these functions more aggressively, as it
does not make sense to use them non-inlined anyway.

[ For reference, the errors look like:

    ld.lld: error: undefined symbol: rust_build_error
    >>> referenced by build_assert.rs:83 (rust/kernel/build_assert.rs:83)
    >>>               rust/doctests_kernel_generated.o:(<kernel::num::bounded::Bounded<u8, 1>>::from_expr) in archive vmlinux.a

      - Miguel ]

Fixes: 01e345e82e ("rust: num: add Bounded integer wrapping type")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202511210055.RUsFNku1-lkp@intel.com/
Suggested-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://patch.msgid.link/20251122-bounded_ints_fix-v1-1-1e07589d4955@nvidia.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/num/bounded.rs

@@ -35,6 +35,7 @@ macro_rules! fits_within {
 }
 
 /// Returns `true` if `value` can be represented with at most `N` bits in a `T`.
+#[inline(always)]
 fn fits_within<T: Integer>(value: T, num_bits: u32) -> bool {
     fits_within!(value, T, num_bits)
 }
@@ -362,6 +363,7 @@ where
     /// assert_eq!(Bounded::<u8, 1>::from_expr(1).get(), 1);
     /// assert_eq!(Bounded::<u16, 8>::from_expr(0xff).get(), 0xff);
     /// ```
+    #[inline(always)]
     pub fn from_expr(expr: T) -> Self {
         crate::build_assert!(
             fits_within(expr, N),