fix behavior does not match name for "IsFileDeletionsEnabled"

Summary:
for PR https://github.com/facebook/rocksdb/pull/3598
I deleted the original repo for some reason. Sorry for the inconvenience.
Closes https://github.com/facebook/rocksdb/pull/3612

Differential Revision: D7291671

Pulled By: ajkr

fbshipit-source-id: 918490ba86b13fe450d232af436cbe259d847c64

db/db_filesnapshot.cc

@@ -74,7 +74,7 @@ Status DBImpl::EnableFileDeletions(bool force) {
 }
 
 int DBImpl::IsFileDeletionsEnabled() const {
-  return disable_delete_obsolete_files_;
+  return !disable_delete_obsolete_files_;
 }
 
 Status DBImpl::GetLiveFiles(std::vector<std::string>& ret,

db/db_properties_test.cc

@@ -69,27 +69,27 @@ TEST_F(DBPropertiesTest, Empty) {
     ASSERT_OK(db_->DisableFileDeletions());
     ASSERT_TRUE(
         dbfull()->GetProperty("rocksdb.is-file-deletions-enabled", &num));
-    ASSERT_EQ("1", num);
+    ASSERT_EQ("0", num);
 
     ASSERT_OK(db_->DisableFileDeletions());
     ASSERT_TRUE(
         dbfull()->GetProperty("rocksdb.is-file-deletions-enabled", &num));
-    ASSERT_EQ("2", num);
+    ASSERT_EQ("0", num);
 
     ASSERT_OK(db_->DisableFileDeletions());
     ASSERT_TRUE(
         dbfull()->GetProperty("rocksdb.is-file-deletions-enabled", &num));
-    ASSERT_EQ("3", num);
+    ASSERT_EQ("0", num);
 
     ASSERT_OK(db_->EnableFileDeletions(false));
     ASSERT_TRUE(
         dbfull()->GetProperty("rocksdb.is-file-deletions-enabled", &num));
-    ASSERT_EQ("2", num);
+    ASSERT_EQ("0", num);
 
     ASSERT_OK(db_->EnableFileDeletions());
     ASSERT_TRUE(
         dbfull()->GetProperty("rocksdb.is-file-deletions-enabled", &num));
-    ASSERT_EQ("0", num);
+    ASSERT_EQ("1", num);
   } while (ChangeOptions());
 }