mount: use fflush() and temporary file for mtab updates (CVE-2011-1089)

mount: use fflush() and temporary file for mtab updates (CVE-2011-1089)

http://thread.gmane.org/gmane.comp.security.oss.general/4374

Changes:

  - force mount(8) to use /etc/mtab.tmp file every time. The original
    code used the tmp file for remount/move operations only.

  - call and check fflush() return code for the tmp file

Note mount(8) blocks all signals when writing to mtab, so it's not
affected by SIGXFSZ and the mtab lock file is always removed.

This patch does not fix the same issue in umount(8) and libmount.

Signed-off-by: Karel Zak <kzak@redhat.com>