mm: clean up and kernelify shrinker registration

I can never remember what the function to register to receive VM pressure
is called.  I have to trace down from __alloc_pages() to find it.

It's called "set_shrinker()", and it needs Your Help.

1) Don't hide struct shrinker.  It contains no magic.
2) Don't allocate "struct shrinker".  It's not helpful.
3) Call them "register_shrinker" and "unregister_shrinker".
4) Call the function "shrink" not "shrinker".
5) Reduce the 17 lines of waffly comments to 13, but document it properly.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: David Chinner <dgc@sgi.com>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index a2b1af8..adffe16 100644
--- a/fs/nfs/super.c
+++ b/fs/nfs/super.c
@@ -300,7 +300,10 @@
 };
 #endif
 
-static struct shrinker *acl_shrinker;
+static struct shrinker acl_shrinker = {
+	.shrink		= nfs_access_cache_shrinker,
+	.seeks		= DEFAULT_SEEKS,
+};
 
 /*
  * Register the NFS filesystems
@@ -321,7 +324,7 @@
 	if (ret < 0)
 		goto error_2;
 #endif
-	acl_shrinker = set_shrinker(DEFAULT_SEEKS, nfs_access_cache_shrinker);
+	register_shrinker(&acl_shrinker);
 	return 0;
 
 #ifdef CONFIG_NFS_V4
@@ -339,8 +342,7 @@
  */
 void __exit unregister_nfs_fs(void)
 {
-	if (acl_shrinker != NULL)
-		remove_shrinker(acl_shrinker);
+	unregister_shrinker(&acl_shrinker);
 #ifdef CONFIG_NFS_V4
 	unregister_filesystem(&nfs4_fs_type);
 	nfs_unregister_sysctl();