mount: improve chmod & chown usage and clean up gcc warnings (fstab.c)

mount: improve chmod & chown usage and clean up gcc warnings (fstab.c)

Fix strict gcc warnings in tailf that come from using:
  ("-Wall -Wp,-D_FORTIFY_SOURCE=2")

fstab.c:770: warning: ignoring return value of 'chown', declared with attribute warn_unused_result

The patch makes chmod() and chown() mandatory. We cannot rename()
temporary mtab to the final mtab when owner is not the same user as
owner of the original mtab. It's security risk.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Karel Zak <kzak@redhat.com>