fat: avoid parent link count underflow in rmdir

In the Linux kernel, the following vulnerability has been resolved: fat: avoid parent link count underflow in rmdir Corrupted FAT images can leave a directory inode with an incorrect i_nlink (e.g. 2 even though subdirectories exist). rmdir then unconditionally calls drop_nlink(dir) and can drive i_nlink to 0, triggering the WARN_ON in drop_nlink(). Add a sanity check in vfat_rmdir() and msdos_rmdir(): only drop the parent link count when it is at least 3, otherwise report a filesystem error.