NFSD: Convert filecache to rhltable

While we were converting the nfs4_file hashtable to use the kernel's
resizable hashtable data structure, Neil Brown observed that the
list variant (rhltable) would be better for managing nfsd_file items
as well. The nfsd_file hash table will contain multiple entries for
the same inode -- these should be kept together on a list. And, it
could be possible for exotic or malicious client behavior to cause
the hash table to resize itself on every insertion.

A nice simplification is that rhltable_lookup() can return a list
that contains only nfsd_file items that match a given inode, which
enables us to eliminate specialized hash table helper functions and
use the default functions provided by the rhashtable implementation).

Since we are now storing nfsd_file items for the same inode on a
single list, that effectively reduces the number of hash entries
that have to be tracked in the hash table. The mininum bucket count
is therefore lowered.

Light testing with fstests generic/531 show no regressions.
Suggested-by: Neil Brown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/filecache.h

@@ -29,9 +29,8 @@ struct nfsd_file_mark {
  * never be dereferenced, only used for comparison.
  */
 struct nfsd_file {
-	struct rhash_head	nf_rhash;
-	struct list_head	nf_lru;
-	struct rcu_head		nf_rcu;
+	struct rhlist_head	nf_rlist;
+	void			*nf_inode;
 	struct file		*nf_file;
 	const struct cred	*nf_cred;
 	struct net		*nf_net;
@@ -40,10 +39,12 @@ struct nfsd_file {
 #define NFSD_FILE_REFERENCED	(2)
 #define NFSD_FILE_GC		(3)
 	unsigned long		nf_flags;
-	struct inode		*nf_inode;	/* don't deref */
 	refcount_t		nf_ref;
 	unsigned char		nf_may;
+
 	struct nfsd_file_mark	*nf_mark;
+	struct list_head	nf_lru;
+	struct rcu_head		nf_rcu;
 	ktime_t			nf_birthtime;
 };