defrag: loosen input file requirements

We don't have perfect control of file allocation for these tests;
in some cases we may fail to adequately fragment a file prior to
defragmentation testing, and today that will fail the test.

Attack this on 2 fronts:

1) Explicitly allow fewer extents on one of the input files in
   generic/018 where the allocator has discretion.
2) _notrun rather than _fail if we don't create enough extents;
   this is a defrag test, not an allocator/fragmentation test,
   so just skip the test if we can't create an acceptable file
   for defrag testing.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Acked-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/generic/018.out

@@ -9,7 +9,7 @@ Contiguous file:
 Before: 1
 After: 1
 Write backwards sync, but contiguous - should defrag to 1 extent
-Before: 10
+Before: in_range(5, 10)
 After: 1
 Write backwards sync leaving holes - defrag should do nothing
 Before: 16