[LibOS] Extend locking of mprotect() during checkpointing

Checkpointing may fail if it assumes memory protection flags that
have not been set on the memory, yet. Therefore, make the code
sequence of

  bkeep_mprotect()

  ...

  PalVirtualMemoryProtect()

atomic so that tracked memory protection flags have also been applied to
the memory.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>

libos/src/sys/libos_mmap.c

@@ -306,9 +306,11 @@ long libos_syscall_mprotect(void* addr, size_t length, int prot) {
     /* `bkeep_mprotect` and then `PalVirtualMemoryProtect` is racy, but it's hard to do it properly.
      * On the other hand if this race happens, it means user app is buggy, so not a huge problem. */
 
+    rwlock_read_lock(&checkpoint_lock);
+
     ret = bkeep_mprotect(addr, length, prot, /*is_internal=*/false);
     if (ret < 0) {
-        return ret;
+        goto error;
     }
 
     if (prot & PROT_GROWSDOWN) {
@@ -323,12 +325,11 @@ long libos_syscall_mprotect(void* addr, size_t length, int prot) {
         } else {
             log_warning("Memory that was about to be mprotected was unmapped, your program is "
                         "buggy!");
-            return -ENOTRECOVERABLE;
+            ret = -ENOTRECOVERABLE;
+            goto error;
         }
     }
 
-    rwlock_read_lock(&checkpoint_lock);
-
     ret = PalVirtualMemoryProtect(addr, length, LINUX_PROT_TO_PAL(prot, /*map_flags=*/0));
 
     rwlock_read_unlock(&checkpoint_lock);
@@ -338,6 +339,11 @@ long libos_syscall_mprotect(void* addr, size_t length, int prot) {
     }
 
     return 0;
+
+error:
+    rwlock_read_unlock(&checkpoint_lock);
+
+    return ret;
 }
 
 long libos_syscall_munmap(void* _addr, size_t length) {