Revert mmap changes

Author: newpavlov

src/sys/mman.rs

@@ -425,12 +425,14 @@ pub unsafe fn mmap<F: AsFd>(
     length: NonZeroUsize,
     prot: ProtFlags,
     flags: MapFlags,
-    f: F,
+    f: Option<F>,
     offset: off_t,
 ) -> Result<*mut c_void> {
-    let ptr = addr.map_or(std::ptr::null_mut(), |a| a.get() as *mut c_void);
+    let ptr =
+        addr.map_or(std::ptr::null_mut(), |a| usize::from(a) as *mut c_void);
+
 
-    let fd = f.as_fd().as_raw_fd();
+    let fd = f.map(|f| f.as_fd().as_raw_fd()).unwrap_or(-1);
     let ret =
         libc::mmap(ptr, length.into(), prot.bits(), flags.bits(), fd, offset);