xfs: test scratch device mkfs features

xfs: test scratch device mkfs features

Some filesystem features are not compatible with certain tests, and
so we need to do require tests to determine if they are supported.
Some of these incompatibilities might be a result of optional mkfs
parameters specified in MKFS_OPTIONS. In this case, we don't want
scratch_mkfs_xfs stripping MKFS_OPTIONS and saying the feature is
supported when the next _scratch_mkfs command in the test will
result in a different mkfs command line.

Hence add a "mkfs supported" function to run a non-destructive mkfs
test to determine if the feature is supported by the current test's
mkfs configuration.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>