applypatch: Fix the return type of FreeSpaceForFile().

Prior to this CL, FreeSpaceForFile() was returning `size_t`, which may
overflow on ILP32 when called on a partition with 4GiB+ free space.
Additionally, it was returning static_cast<size_t>(-1) on error, but the
caller in freecache.cpp didn't check for that.

This CL changes its return type to `int64_t`, and moves the function
into freecache.cpp since there's no external caller.

Test: Run recovery_unit_test on marlin.
Test: Code search shows no external user of FreeSpaceForFile().
Change-Id: I00f501a057726e1f1ab69f367c46c77b30f2d774

applypatch/applypatch.cpp

@@ -23,7 +23,6 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
-#include <sys/statfs.h>
 #include <sys/types.h>
 #include <unistd.h>
 
@@ -421,15 +420,6 @@ static size_t FileSink(const unsigned char* data, size_t len, int fd) {
   return done;
 }
 
-size_t FreeSpaceForFile(const std::string& filename) {
-  struct statfs sf;
-  if (statfs(filename.c_str(), &sf) != 0) {
-    PLOG(ERROR) << "Failed to statfs " << filename;
-    return -1;
-  }
-  return sf.f_bsize * sf.f_bavail;
-}
-
 int CacheSizeCheck(size_t bytes) {
   if (MakeFreeSpaceOnCache(bytes) < 0) {
     LOG(ERROR) << "Failed to make " << bytes << " bytes available on /cache";