nfsd4: support putpubfh operation

Currently putpubfh returns NFSERR_OPNOTSUPP, which isn't actually
allowed for v4.  The right error is probably NFSERR_NOTSUPP.

But let's just implement it; though rarely seen, it can be used by
Solaris (with a special mount option), is mandated by the rfc, and is
trivial for us to support.

Thanks to Yang Hongyang for pointing out the original problem, and to
Mike Eisler, Tom Talpey, Trond Myklebust, and Dave Noveck for further
argument....
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4proc.c

@@ -1045,7 +1045,7 @@ static struct nfsd4_operation nfsd4_ops[OP_RELEASE_LOCKOWNER+1] = {
 		.op_name = "OP_PUTFH",
 	},
 	[OP_PUTPUBFH] = {
-		/* unsupported, just for future reference: */
+		.op_func = (nfsd4op_func)nfsd4_putrootfh,
 		.op_flags = ALLOWED_WITHOUT_FH | ALLOWED_ON_ABSENT_FS,
 		.op_name = "OP_PUTPUBFH",
 	},

fs/nfsd/nfs4xdr.c

@@ -1031,7 +1031,7 @@ static nfsd4_dec nfsd4_dec_ops[] = {
 	[OP_OPEN_CONFIRM]	= (nfsd4_dec)nfsd4_decode_open_confirm,
 	[OP_OPEN_DOWNGRADE]	= (nfsd4_dec)nfsd4_decode_open_downgrade,
 	[OP_PUTFH]		= (nfsd4_dec)nfsd4_decode_putfh,
-	[OP_PUTPUBFH]		= (nfsd4_dec)nfsd4_decode_notsupp,
+	[OP_PUTPUBFH]		= (nfsd4_dec)nfsd4_decode_noop,
 	[OP_PUTROOTFH]		= (nfsd4_dec)nfsd4_decode_noop,
 	[OP_READ]		= (nfsd4_dec)nfsd4_decode_read,
 	[OP_READDIR]		= (nfsd4_dec)nfsd4_decode_readdir,