Fix Fadvise on closed file when reads use mmap

Summary:
```PosixMmapReadableFile::fd_``` is closed after created, but needs to remain open for the lifetime of `PosixMmapReadableFile` since it is used whenever `InvalidateCache` is called.
Closes https://github.com/facebook/rocksdb/pull/2764

Differential Revision: D8152515

Pulled By: ajkr

fbshipit-source-id: b738a6a55ba4e392f9b0f374ff396a1e61c64f65

env/env_posix.cc

@@ -241,9 +241,9 @@ class PosixEnv : public Env {
                                                   size, options));
         } else {
           s = IOError("while mmap file for read", fname, errno);
-        }
-      }
           close(fd);
+        }
+      }
     } else {
       if (options.use_direct_reads && !options.use_mmap_reads) {
 #ifdef OS_MACOSX

env/io_posix.cc

@@ -456,6 +456,7 @@ PosixMmapReadableFile::~PosixMmapReadableFile() {
     fprintf(stdout, "failed to munmap %p length %" ROCKSDB_PRIszt " \n",
             mmapped_region_, length_);
   }
+  close(fd_);
 }
 
 Status PosixMmapReadableFile::Read(uint64_t offset, size_t n, Slice* result,