Disable the code which sets the file size on the MDS:

Disable the code which sets the file size on the MDS:
1) we never (or should never) currently use the file size from the MDS
2) only the client which holds the size lock would need to update the MDS
3) that client already knows the correct file size locally, so no obd_getattr

Removing this hunk of code also has the benefit of removing an extraneous
obd_getattr() x stripe_count + mdc_setattr() per file, so it should improve
performance for tar/bonnie/etc noticably.

Passes multi-client runtests.