NFSv4.2: fix listxattr maximum XDR buffer size

Switch order of operations to avoid creating a short XDR buffer:
e.g., buflen = 12, old xdrlen = 12, new xdrlen = 20.

Having a short XDR buffer leads to lxa_maxcount be a few bytes
less than what is needed to retrieve the whole list when using
a buflen as returned by a call with size = 0:
    buflen = listxattr(path, NULL, 0);
    buf = malloc(buflen);
    buflen = listxattr(path, buf, buflen);

For a file with one attribute (name = '123456'), the first call
with size = 0 will return buflen = 12 ('user.123456\x00').
The second call with size = 12, sends LISTXATTRS with
lxa_maxcount = 12 + 8 (cookie) + 4 (array count) = 24. The
XDR buffer needs 8 (cookie) + 4 (array count) + 4 (name count)
+ 6 (name len) + 2 (padding) + 4 (eof) = 28 which is 4 bytes
shorter than the lxa_maxcount provided in the call.

Fixes: 04a5da690e ("NFSv4.2: define limits and sizes for user xattr handling")
Signed-off-by: Jorge Mora <mora@netapp.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/nfs42.h

@@ -55,11 +55,14 @@ int nfs42_proc_removexattr(struct inode *inode, const char *name);
  * They would be 7 bytes long in the eventual buffer ("user.x\0"), and
  * 8 bytes long XDR-encoded.
  *
- * Include the trailing eof word as well.
+ * Include the trailing eof word as well and make the result a multiple
+ * of 4 bytes.
  */
 static inline u32 nfs42_listxattr_xdrsize(u32 buflen)
 {
-	return ((buflen / (XATTR_USER_PREFIX_LEN + 2)) * 8) + 4;
+	u32 size = 8 * buflen / (XATTR_USER_PREFIX_LEN + 2) + 4;
+
+	return (size + 3) & ~3;
 }
 #endif /* CONFIG_NFS_V4_2 */
 #endif /* __LINUX_FS_NFS_NFS4_2_H */