Use MAP_SHARED for shared memory creation again

Previously, in 7ca0a78, MAP_SHARED was replaced with MAP_PRIVATE,
primarily to bring the Miri and non-Miri code paths closer together,
since it did not appear to have any detrimental effect.  However, this
seems to cause issues with the scratch mapping on mshv on Linux. It is
not entirely clear what mshv is doing here; possibly something like
"mapping a page which has never been written to" causes the problem.

Signed-off-by: Lucy Menon <168595099+syntactically@users.noreply.github.com>

Author: syntactically

src/hyperlight_host/src/mem/shared_mem.rs

@@ -314,12 +314,11 @@ impl ExclusiveSharedMemory {
     #[cfg(target_os = "linux")]
     #[instrument(skip_all, parent = Span::current(), level= "Trace")]
     pub fn new(min_size_bytes: usize) -> Result<Self> {
-        use libc::{
-            MAP_ANONYMOUS, MAP_FAILED, MAP_PRIVATE, PROT_READ, PROT_WRITE, c_int, mmap, off_t,
-            size_t,
-        };
+        #[cfg(miri)]
+        use libc::MAP_PRIVATE;
+        use libc::{MAP_ANONYMOUS, MAP_FAILED, PROT_READ, PROT_WRITE, c_int, mmap, off_t, size_t};
         #[cfg(not(miri))]
-        use libc::{MAP_NORESERVE, PROT_NONE, mprotect};
+        use libc::{MAP_NORESERVE, MAP_SHARED, PROT_NONE, mprotect};
 
         if min_size_bytes == 0 {
             return Err(new_error!("Cannot create shared memory with size 0"));
@@ -347,7 +346,7 @@ impl ExclusiveSharedMemory {
 
         // allocate the memory
         #[cfg(not(miri))]
-        let flags = MAP_ANONYMOUS | MAP_PRIVATE | MAP_NORESERVE;
+        let flags = MAP_ANONYMOUS | MAP_SHARED | MAP_NORESERVE;
         #[cfg(miri)]
         let flags = MAP_ANONYMOUS | MAP_PRIVATE;