Stop doing full fsck on private f2fs partitions

This is safe because we run 'f2fs.fsck -f' on all writable
/dev/block* paritions on shutdown.

Bug: 21853106
Change-Id: I368a8676093c97e70ad7c09e9b71665d3c445a16
diff --git a/fs/F2fs.cpp b/fs/F2fs.cpp
index b947822..87b131e 100644
--- a/fs/F2fs.cpp
+++ b/fs/F2fs.cpp
@@ -43,7 +43,7 @@
 status_t Check(const std::string& source) {
     std::vector<std::string> cmd;
     cmd.push_back(kFsckPath);
-    cmd.push_back("-f");
+    cmd.push_back("-a");
     cmd.push_back(source);
 
     // f2fs devices are currently always trusted