nfsd4: simplify reading of opnum

The comment here is totally bogus:
	- OP_WRITE + 1 is RELEASE_LOCKOWNER.  Maybe there was some older
	  version of the spec in which that served as a sort of
	  OP_ILLEGAL?  No idea, but it's clearly wrong now.
	- In any case, I can't see that the spec says anything about
	  what to do if the client sends us less ops than promised.
	  It's clearly nutty client behavior, and we should do
	  whatever's easiest: returning an xdr error (even though it
	  won't be consistent with the error on the last op returned)
	  seems fine to me.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>