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>

fs/inode.c

@@ -462,6 +462,11 @@ static int shrink_icache_memory(int nr, gfp_t gfp_mask)
 	return (inodes_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
 }
 
+static struct shrinker icache_shrinker = {
+	.shrink = shrink_icache_memory,
+	.seeks = DEFAULT_SEEKS,
+};
+
 static void __wait_on_freeing_inode(struct inode *inode);
 /*
  * Called with the inode lock held.
@@ -1385,7 +1390,7 @@ void __init inode_init(unsigned long mempages)
 					 SLAB_MEM_SPREAD),
 					 init_once,
 					 NULL);
-	set_shrinker(DEFAULT_SEEKS, shrink_icache_memory);
+	register_shrinker(&icache_shrinker);
 
 	/* Hash may have been set up in inode_init_early */
 	if (!hashdist)