By default, FUSE file systems are assumed not to support lookups for "."
and "..". They must opt-in to that. To cope with this limitation, the
fusefs kernel module caches every fuse vnode's parent's inode number,
and uses that during VOP_LOOKUP for "..". But if the parent's vnode has
been reclaimed that won't be possible. Previously we paniced in this
situation. Now, we'll return ESTALE instead. Or, if the file system
has opted into ".." lookups, we'll just do that instead.
This commit also fixes VOP_LOOKUP to respect the cache timeout for ".."
lookups, if the FUSE file system specified a finite timeout.
PR: 259974
MFC after: 2 weeks
Reviewed by: pfg
Differential Revision: https://reviews.freebsd.org/D33239
1613087a81