mount: should set proper permissions on locktime

mount: should set proper permissions on locktime

When creating the "/etc/mtab~" lockfile (specifically 'linktargetfile'  in the
lock_mtab function), the file is created with incorrect permissions ('000')
which necessitates root to leverage CAP_DAC_OVERRIDE. If proper file modes (it
would appear 0600 would be sufficient) were used in the open this would
function properly with CAP_DAC_OVERRIDE revoked.

  $ sysctl -w kernel.cap-bound=0xf7fd7df5
  $ mount -t tmpfs /dev/swap /mnt
  can't open lock file /etc/mtab~: Permission denied (use -n flag to override)

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