JSBigString to MAP_PRIVATE not MAP_SHARED

Reviewed By: mhorowitz

Differential Revision: D6613463

fbshipit-source-id: 2553a0475ade061f3b70ffce90fcf4e0ceb7d9fa

Author: Peter Ammon

Diff for: ReactCommon/cxxreact/JSBigString.h

@@ -146,7 +146,8 @@ class RN_EXPORT JSBigFileString : public JSBigString {
 
   const char *c_str() const override {
     if (!m_data) {
-      m_data = (const char *)mmap(0, m_size, PROT_READ, MAP_SHARED, m_fd, m_mapOff);
+      m_data =
+        (const char *)mmap(0, m_size, PROT_READ, MAP_PRIVATE, m_fd, m_mapOff);
       CHECK(m_data != MAP_FAILED)
       << " fd: " << m_fd
       << " size: " << m_size