nfsd: add some comments to nfsd_file_do_acquire

David Howells mentioned that he found this bit of code confusing, so
sprinkle in some comments to clarify.
Reported-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>

fs/nfsd/filecache.c

@@ -1094,6 +1094,11 @@ nfsd_file_do_acquire(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	rcu_read_unlock();
 
 	if (nf) {
+		/*
+		 * If the nf is on the LRU then it holds an extra reference
+		 * that must be put if it's removed. It had better not be
+		 * the last one however, since we should hold another.
+		 */
 		if (nfsd_file_lru_remove(nf))
 			WARN_ON_ONCE(refcount_dec_and_test(&nf->nf_ref));
 		goto wait_for_construction;