fusefs: fix error handling when reading a directory's sticky bit

[asomers]

Summary

When trying to delete or rename a file, fuse_vnop_lookup must check
whether its parent directory's sticky bit is set. Realistically, the
parent directory's attributes will almost always be cached. But it's
possible that they won't be, and in that case we must send a new
FUSE_GETATTR request to the server. If that request fails for some
reason, then we must fail the lookup. Prior to this change fusefs would
ignore failure of that request.

Reported by: Yuxiang Yang, Yizhou Zhao, Ao Wang, Xuewei Feng, Qi Li,

and Ke Xu of Tsinghua University

MFC after: 2 weeks

Test Plan

Test case added