ext4: allow a NULL argument to ext4_ext_drop_refs()

Teach ext4_ext_drop_refs() to accept a NULL argument, much like
kfree().  This allows us to drop a lot of checks to make sure path is
non-NULL before calling ext4_ext_drop_refs().

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
diff --git a/fs/ext4/migrate.c b/fs/ext4/migrate.c
index aff7bdf..061c300 100644
--- a/fs/ext4/migrate.c
+++ b/fs/ext4/migrate.c
@@ -84,10 +84,8 @@
 	retval = ext4_ext_insert_extent(handle, inode, &path, &newext, 0);
 err_out:
 	up_write((&EXT4_I(inode)->i_data_sem));
-	if (path) {
-		ext4_ext_drop_refs(path);
-		kfree(path);
-	}
+	ext4_ext_drop_refs(path);
+	kfree(path);
 	lb->first_pblock = 0;
 	return retval;
 }