xfstests 249: use -F option for xfs_io

xfstests 249: use -F option for xfs_io

Test 249 was appearing to pass on ext4, but it wasn't really
exercising the test due to lack of "-F" in the xfs_io arguments.

Without -F the files were created (oddly enough); neither pwrite
nor sendfile were executed, and the diff of the two (empty)
files passed, resulting in a passed test without testing anything.

So add the -F, capture the output, and test the result of each
xfs_io invocation.

Also, when it fails, the diff output is huge.  Make diff silent,
but describe the diff failure and exit.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>