xfstests: add mapped write fsx operations to 091

xfstests: add mapped write fsx operations to 091

The recent busted fsx updates caused fsx to execute fsx with direct
IO and mmapped reads and writes on an XFS filesystem. The result
uncovered a direct-IO write vs mmap read bug to do with EOF
sub-block zeroing on the direct IO write.

Hence whiel we do not recommend that pepole mix DIO with mmap on the
same file, we should at least have tests that exercise it as they
often show up other problems like this.

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