MD linear/0/1/10/5 TRIM support

This is the patchset to support MD discard, which I refresh against latest
kernel. It's pretty straightforward for raid linear/0/1/10. The raid456 discard
support is tricky, please see the log in the patches for details.

Previously when I posted the patches out, people complain SATA SSD discard
request merge error. Now a patch to disable discard request merge is in Jens's
tree. We don't have this issue any more.
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