fs-tests: integck: prefer to check for success

fs-tests: integck: prefer to check for success

Many of standard C libraries return 0 on success and -1 on failure.
Currently integck mostly checks for failure by comparing with -1, like:

CHECK(fsync(fd) != -1) /* Don't die if not failure */

but in some places the check is like

CHECK(fsync(fd) == 0) /* Don't die if success */

This patch harmonizes this an makes integck to use the second style
which seems to be more logical. Besides, the binary is smaller in
this case:

[dedekind@eru fs-tests (master)]$ size integrity/integck-old
   text    data     bss     dec     hex filename
  44677    1200   37408   83285   14555 integrity/integck
[dedekind@eru fs-tests (master)]$ size integrity/integck-new
   text    data     bss     dec     hex filename
  44661    1200   37408   83269   14545 integrity/integck

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>