nfs: don't invalidate dentries on transient errors

This is a slight variation on a patch previously proposed by Neil Brown
that never got merged.

Prior to commit 5ceb9d7f ("NFS: Refactor nfs_lookup_revalidate()"),
any error from nfs_lookup_verify_inode() other than -ESTALE would result
in nfs_lookup_revalidate() returning that error (-ESTALE is mapped to
zero).

Since that commit, all errors result in nfs_lookup_revalidate()
returning zero, resulting in dentries being invalidated where they
previously were not (particularly in the case of -ERESTARTSYS).

Fix it by passing the actual error code to nfs_lookup_revalidate_done(),
and leaving the decision on whether to  map the error code to zero or
one to nfs_lookup_revalidate_done().

A simple reproducer is to run the following python code in a
subdirectory of an NFS mount (not in the root of the NFS mount):

---8<---
import os
import multiprocessing
import time

if __name__=="__main__":
    multiprocessing.set_start_method("spawn")

    count = 0
    while True:
        try:
            os.getcwd()
            pool = multiprocessing.Pool(10)
            pool.close()
            pool.terminate()
            count += 1
        except Exception as e:
            print(f"Failed after {count} iterations")
            print(e)
            break
---8<---

Prior to commit 5ceb9d7f, the above code would run indefinitely.
After commit 5ceb9d7f, it fails almost immediately with -ENOENT.

Signed-off-by: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>