NFSD: Avoid clashing function prototypes

When built with Control Flow Integrity, function prototypes between
caller and function declaration must match. These mismatches are visible
at compile time with the new -Wcast-function-type-strict in Clang[1].

There were 97 warnings produced by NFS. For example:

fs/nfsd/nfs4xdr.c:2228:17: warning: cast from '__be32 (*)(struct nfsd4_compoundargs *, struct nfsd4_access *)' (aka 'unsigned int (*)(struct nfsd4_compoundargs *, struct nfsd4_access *)') to 'nfsd4_dec' (aka 'unsigned int (*)(struct nfsd4_compoundargs *, void *)') converts to incompatible function type [-Wcast-function-type-strict]
        [OP_ACCESS]             = (nfsd4_dec)nfsd4_decode_access,
                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The enc/dec callbacks were defined as passing "void *" as the second
argument, but were being implicitly cast to a new type. Replace the
argument with union nfsd4_op_u, and perform explicit member selection
in the function body. There are no resulting binary differences.

Changes were made mechanically using the following Coccinelle script,
with minor by-hand fixes for members that didn't already match their
existing argument name:

@find@
identifier func;
type T, opsT;
identifier ops, N;
@@

 opsT ops[] = {
	[N] = (T) func,
 };

@already_void@
identifier find.func;
identifier name;
@@

 func(...,
-void
+union nfsd4_op_u
 *name)
 {
	...
 }

@proto depends on !already_void@
identifier find.func;
type T;
identifier name;
position p;
@@

 func@p(...,
 	T name
 ) {
	...
   }

@script:python get_member@
type_name << proto.T;
member;
@@

coccinelle.member = cocci.make_ident(type_name.split("_", 1)[1].split(' ',1)[0])

@convert@
identifier find.func;
type proto.T;
identifier proto.name;
position proto.p;
identifier get_member.member;
@@

 func@p(...,
-	T name
+	union nfsd4_op_u *u
 ) {
+	T name = &u->member;
	...
   }

@cast@
identifier find.func;
type T, opsT;
identifier ops, N;
@@

 opsT ops[] = {
	[N] =
-	(T)
	func,
 };

Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Cc: linux-nfs@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>