generic/017: Do not create file systems with different block sizes

generic/017: Do not create file systems with different block sizes

User takes care about specifying mkfs options he wishes to test and the
test itself should not change it if it's not strictly necessary for the
test itself.

In this case it is not necessary and we should only test configuration
provided by the user. Moreover if the block size was already specified
some mkfs utilities does not handle multiple of the same parameters and
the mkfs utility fails making it re-try with only provided options
(ignoring what user specified), which is wrong.

In this case it's also a problem for btrfs file system which does not
support block size < page size.

Fix it by removing the mkfs, and testing existing configuration only.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>