LINUX: Restore negative dentry caching

LINUX: Restore negative dentry caching

One of the changes in commit 652f3bd9cb7a5d7833a760ba50ef7c2c67214bba
effectively disabled negative caching for dentries, by always
invalidating a negative dentry in afs_linux_dentry_revalidate. This
was because various temporary errors could result in ENOENT errors
being returned to afs_lookup, which created incorrect negative dentry
cache entries.

These incorrect ENOENT errors were rectified in change
Ib01e4309e44b532f843d53c8de2eae613e397bf6 . So, negative dentry cache
entries should work now, so remove the code to unconditionally
invalidate these negative entries.

Change-Id: Ic027147fd1f733beaa0fafbbabfa8c09f5656d34
Reviewed-on: http://gerrit.openafs.org/11789
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>