sys/miri: fix Mmap::from_file to seek to start of file. (#12572)

The constructor takes a `&File` but `&File` still allows mutation of
the underlying state, e.g., the seek position. It turns out that this
matters when we try to `Mmap::from_file` more than once, e.g. when
enabling guest debugging and thus mapping private copies of code.

No test yet as this will be covered by upcoming miri coverage of guest
debugging.

Author: cfallin

crates/wasmtime/src/runtime/vm/sys/miri/mmap.rs

@@ -10,7 +10,7 @@ use crate::runtime::vm::sys::vm::MemoryImageSource;
 use crate::runtime::vm::{HostAlignedByteCount, SendSyncPtr};
 use std::alloc::{self, Layout};
 use std::fs::File;
-use std::io::Read;
+use std::io::{Read, Seek, SeekFrom};
 use std::ops::Range;
 use std::path::Path;
 use std::ptr::NonNull;
@@ -63,6 +63,7 @@ impl Mmap {
         // Read the file and copy it in to a fresh "mmap" to have allocation for
         // an mmap only in one location.
         let mut dst = Vec::new();
+        file.seek(SeekFrom::Start(0))?;
         file.read_to_end(&mut dst)?;
         let count = HostAlignedByteCount::new_rounded_up(dst.len())?;
         let result = Mmap::new(count)?;