f2fs: correctly fix the parent inode number during fsync()

fsync() may be called on a deleted file that's still open.  So when
fsync() tries to set the parent inode number when the inode has
LOST_PINO and i_nlink == 1 (to avoid later checkpoints), it needs to
make sure to get the parent directory via a non-deleted alias.

Also remove the unnecessary igrab() and iput(), as the caller already
holds a reference to the inode.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -169,9 +169,11 @@ static int get_parent_ino(struct inode *inode, nid_t *pino)
 {
 	struct dentry *dentry;
 
-	inode = igrab(inode);
-	dentry = d_find_any_alias(inode);
-	iput(inode);
+	/*
+	 * Make sure to get the non-deleted alias.  The alias associated with
+	 * the open file descriptor being fsync()'ed may be deleted already.
+	 */
+	dentry = d_find_alias(inode);
 	if (!dentry)
 		return 0;