fix: Fix sign-compare warning src/InodeCache.cpp on FreeBSD (#1331)

fix: Fix sign-compare warning src/InodeCache.cpp on FreeBSD (#1331)

As on FreeBSD 13.2 statfs.f_bavail is signed, InodeCache.cpp cannot be compiled
but gets a sign-compare warning:

  src/InodeCache.cpp:409:30: error: comparison of integers of different signs:
    'long' and 'unsigned long' [-Werror,-Wsign-compare]
      if (buf.f_bavail * 512 < k_min_fs_mib_left * 1024 * 1024) {
          ~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

The problem is avoided by using static_cast to uint64_t.