GFS2: fix d_splice_alias() misuses

Callers of d_splice_alias(dentry, inode) don't need iput(), neither
on success nor on failure.  Either the reference to inode is stored
in a previously negative dentry, or it's dropped.  In either case
inode reference the caller used to hold is consumed.

__gfs2_lookup() does iput() in case when d_splice_alias() has failed.
Double iput() if we ever hit that.  And gfs2_create_inode() ends up
not only with double iput(), but with link count dropped to zero - on
an inode it has just found in directory.

Cc: stable@vger.kernel.org # v3.14+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/inode.c

@@ -626,8 +626,10 @@ static int gfs2_create_inode(struct inode *dir, struct dentry *dentry,
 	if (!IS_ERR(inode)) {
 		d = d_splice_alias(inode, dentry);
 		error = PTR_ERR(d);
-		if (IS_ERR(d))
+		if (IS_ERR(d)) {
+			inode = ERR_CAST(d);
 			goto fail_gunlock;
+		}
 		error = 0;
 		if (file) {
 			if (S_ISREG(inode->i_mode)) {
@@ -856,7 +858,6 @@ static struct dentry *__gfs2_lookup(struct inode *dir, struct dentry *dentry,
 
 	d = d_splice_alias(inode, dentry);
 	if (IS_ERR(d)) {
-		iput(inode);
 		gfs2_glock_dq_uninit(&gh);
 		return d;
 	}