f2fs: fix args passed to trace_f2fs_lookup_end

The NULL return of 'd_splice_alias' dosen't mean error. Thus the
successful case will also return NULL, which makes the tracepoint always
print 'err=-ENOENT'.

And the different cases of 'new' & 'err' are list as following:
1) dentry exists: err(0) with new(NULL) --> dentry, err=0
2) dentry exists: err(0) with new(VALID) --> new, err=0
3) dentry exists: err(0) with new(ERR) --> dentry, err=ERR
4) no dentry exists: err(-ENOENT) with new(NULL) --> dentry, err=-ENOENT
5) no dentry exists: err(-ENOENT) with new(VALID) --> new, err=-ENOENT
6) no dentry exists: err(-ENOENT) with new(ERR) --> dentry, err=ERR

Signed-off-by: Wu Bo <bo.wu@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: Wu Bo

fs/f2fs/namei.c

@@ -576,8 +576,8 @@ static struct dentry *f2fs_lookup(struct inode *dir, struct dentry *dentry,
 	}
 #endif
 	new = d_splice_alias(inode, dentry);
-	err = PTR_ERR_OR_ZERO(new);
-	trace_f2fs_lookup_end(dir, dentry, ino, !new ? -ENOENT : err);
+	trace_f2fs_lookup_end(dir, !IS_ERR_OR_NULL(new) ? new : dentry,
+				ino, IS_ERR(new) ? PTR_ERR(new) : err);
 	return new;
 out_iput:
 	iput(inode);