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.