xfstests: test EROFS vs. EEXIST when creating on an RO filesystem

xfstests: test EROFS vs. EEXIST when creating on an RO filesystem

TBH, I don't know if this is posix-specified, but I found out the
hard way that when trying to re-create existing files on a readonly
filesystem, some apps expect/handle EEXIST, but fail on EROFS.

This will test mkdir, mknod, and symlinks for that behavior.

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