std: sys: fs: uefi: Implement File::tell

- Just a call to get_position
- Tested with OVMF on QEMU

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

library/std/src/sys/fs/uefi.rs

@@ -364,7 +364,7 @@ impl File {
     }
 
     pub fn tell(&self) -> io::Result<u64> {
-        unsupported()
+        self.0.position()
     }
 
     pub fn duplicate(&self) -> io::Result<File> {
@@ -734,6 +734,14 @@ mod uefi_fs {
             if r.is_error() { Err(io::Error::from_raw_os_error(r.as_usize())) } else { Ok(()) }
         }
 
+        pub(crate) fn position(&self) -> io::Result<u64> {
+            let file_ptr = self.protocol.as_ptr();
+            let mut pos = 0;
+
+            let r = unsafe { ((*file_ptr).get_position)(file_ptr, &mut pos) };
+            if r.is_error() { Err(io::Error::from_raw_os_error(r.as_usize())) } else { Ok(pos) }
+        }
+
         pub(crate) fn delete(self) -> io::Result<()> {
             let file_ptr = self.protocol.as_ptr();
             let r = unsafe { ((*file_ptr).delete)(file_ptr) };