vfs: move d_lockref out of the area used by RCU lookup

Stock kernel scales worse than FreeBSD when doing a 20-way stat(2) on
the same tmpfs-backed file.

According to perf top:
  38.09%  lockref_put_return
  26.08%  lockref_get_not_dead
  25.60%  __d_lookup_rcu
   0.89%  clear_bhb_loop

__d_lookup_rcu is participating in cacheline ping pong due to the
embedded name sharing a cacheline with lockref.

Moving it out resolves the problem:
  41.50%  lockref_put_return
  41.03%  lockref_get_not_dead
   1.54%  clear_bhb_loop

benchmark (will-it-scale, Sapphire Rapids, tmpfs, ops/s):
FreeBSD:7219334
before:	5038006
after:	7842883 (+55%)

One minor remark: the 'after' result is unstable, fluctuating in the
range ~7.8 mln to ~9 mln during different runs.
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240613001215.648829-3-mjguzik@gmail.comSigned-off-by: Christian Brauner <brauner@kernel.org>

include/linux/dcache.h

@@ -89,13 +89,18 @@ struct dentry {
 	struct inode *d_inode;		/* Where the name belongs to - NULL is
 					 * negative */
 	unsigned char d_iname[DNAME_INLINE_LEN];	/* small names */
+	/* --- cacheline 1 boundary (64 bytes) was 32 bytes ago --- */
 
 	/* Ref lookup also touches following */
-	struct lockref d_lockref;	/* per-dentry lock and refcount */
 	const struct dentry_operations *d_op;
 	struct super_block *d_sb;	/* The root of the dentry tree */
 	unsigned long d_time;		/* used by d_revalidate */
 	void *d_fsdata;			/* fs-specific data */
+	/* --- cacheline 2 boundary (128 bytes) --- */
+	struct lockref d_lockref;	/* per-dentry lock and refcount
+					 * keep separate from RCU lookup area if
+					 * possible!
+					 */
 
 	union {
 		struct list_head d_lru;		/* LRU list */