Use reason specifier in allow

Signed-off-by: mulhern <amulhern@redhat.com>

Author: mulkieran

src/devno.rs

@@ -20,7 +20,7 @@ impl BlkidDevno {
 
     /// Create a `BlkidDevno` from major and minor numbers.
     pub fn from_device_numbers(major: libc::c_uint, minor: libc::c_uint) -> Self {
-        #[allow(unused_unsafe)] // No longer unsafe in libc 0.2.133
+        #[allow(unused_unsafe, reason = "No longer unsafe in libc 0.2.133")]
         BlkidDevno(unsafe { libc::makedev(major, minor) })
     }