jbd2: preserve original nofs flag during journal restart

When a transaction starts, start_this_handle() saves current
PF_MEMALLOC_NOFS value so that it can be restored at journal stop time.
Journal restart is a special case that calls start_this_handle() without
stopping the transaction. start_this_handle() isn't aware that the
original value is already stored so it overwrites it with current value.

For instance, a call sequence like below leaves PF_MEMALLOC_NOFS flag set
at the end:

  jbd2_journal_start()
  jbd2__journal_restart()
  jbd2_journal_stop()

Make jbd2__journal_restart() restore the original value before calling
start_this_handle().

Fixes: 81378da64de6 ("jbd2: mark the transaction context with the scope GFP_NOFS context")
Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

fs/jbd2/transaction.c

@@ -680,6 +680,12 @@ int jbd2__journal_restart(handle_t *handle, int nblocks, gfp_t gfp_mask)
 
 	rwsem_release(&journal->j_trans_commit_map, 1, _THIS_IP_);
 	handle->h_buffer_credits = nblocks;
+	/*
+	 * Restore the original nofs context because the journal restart
+	 * is basically the same thing as journal stop and start.
+	 * start_this_handle will start a new nofs context.
+	 */
+	memalloc_nofs_restore(handle->saved_alloc_context);
 	ret = start_this_handle(journal, handle, gfp_mask);
 	return ret;
 }