fs: implement FromRawFd for File (#89)

Implement FromRawFd for File, so we could convert a normal file into
tokio-uring File.

Signed-off-by: Jiang Liu <gerry@linux.alibaba.com>

src/fs/file.rs

@@ -4,7 +4,7 @@ use crate::fs::OpenOptions;
 
 use std::fmt;
 use std::io;
-use std::os::unix::io::{AsRawFd, RawFd};
+use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 use std::path::Path;
 
 /// A reference to an open file on the filesystem.
@@ -330,6 +330,12 @@ impl File {
     }
 }
 
+impl FromRawFd for File {
+    unsafe fn from_raw_fd(fd: RawFd) -> Self {
+        File::from_shared_fd(SharedFd::new(fd))
+    }
+}
+
 impl AsRawFd for File {
     fn as_raw_fd(&self) -> RawFd {
         self.fd.raw_fd()