rust: bug: fix warn_on macro build error on UML

Callers that go through `kernel::prelude` have `CStrExt` in scope, but
code inside the `kernel` crate imports explicitly and may not. Using
`warn_on!` from such a module fails to build on UML, which is the only
configuration where `warn_flags!` needs a C string pointer rather than an
inline asm bug entry:

error[E0599]: no method named `as_char_ptr` found for reference `&ffi::CStr` in the current scope
   --> linux/rust/kernel/bug.rs:83:49
    |
 83 |                 $crate::c_str!(::core::file!()).as_char_ptr(),
    |                                                 ^^^^^^^^^^^
    |
   ::: linux/rust/kernel/time.rs:427:9
    |
427 |         warn_on!(self.nanos < 0);
    |         ------------------------ in this macro invocation
    |
    = help: items from traits can only be used if the trait is in scope
    = note: this error originates in the macro `$crate::warn_flags` which comes from the expansion of the macro `warn_on` (in Nightly builds, run with -Z mac)
help: trait `CStrExt` which provides `as_char_ptr` is implemented but not in scope; perhaps you want to import it
   --> linux/rust/kernel/time.rs:27:1
    |
 27 + use crate::str::CStrExt;

Call the method through its fully qualified path, which resolves
without any import at the expansion site.

Cc: stable@vger.kernel.org
Fixes: dff64b0727 ("rust: Add warn_on macro")
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Link: https://patch.msgid.link/20260807112427.1039056-1-tomo@flapping.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/bug.rs

@@ -91,7 +91,7 @@ macro_rules! warn_flags {
         // with a valid null-terminated string.
         unsafe {
             $crate::bindings::warn_slowpath_fmt(
-                $crate::c_str!(::core::file!()).as_char_ptr(),
+                $crate::str::CStrExt::as_char_ptr($crate::c_str!(::core::file!())),
                 line!() as $crate::ffi::c_int,
                 $flags as $crate::ffi::c_uint,
                 ::core::ptr::null(),