nfsd4: bad BUG() in preprocess_stateid_op

It's OK for this function to return without setting filp--we do it in
the special-stateid case.

And there's a legitimate case where we can hit this, since we do permit
reads on write-only stateid's.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -2983,7 +2983,6 @@ nfs4_preprocess_stateid_op(struct nfsd4_compound_state *cstate,
 				*filpp = find_readable_file(stp->st_file);
 			else
 				*filpp = find_writeable_file(stp->st_file);
-			BUG_ON(!*filpp); /* assured by check_openmode */
 		}
 	}
 	status = nfs_ok;