aya-obj: mask Func linkage with 16 bits

Match `Func::new` and `set_linkage`, which store linkage in the low
16 bits of `info`; reading with 0xFFF dropped the high nibble of that
field.

aya-obj/src/btf/types.rs

@@ -271,7 +271,7 @@ impl Func {
     }
 
     pub(crate) fn linkage(&self) -> FuncLinkage {
-        (self.info & 0xFFF).into()
+        (self.info & 0xFFFF).into()
     }
 
     pub(crate) const fn set_linkage(&mut self, linkage: FuncLinkage) {