Grub2: grub2-set-bootflag can be abused by local (pseudo-)users

A flaw was found in the grub2-set-bootflag utility of grub2. After the fix of CVE-2019-14865, grub2-set-bootflag will create a temporary file with the new grubenv content and rename it to the original grubenv file. If the program is killed before the rename operation, the temporary file will not be removed and may fill the filesystem when invoked multiple times, resulting in a filesystem out of free inodes or blocks.