fs: use percpu counter for nr_dentry and nr_dentry_unused

The nr_dentry stat is a globally touched cacheline and atomic operation
twice over the lifetime of a dentry. It is used for the benfit of userspace
only. Turn it into a per-cpu counter and always decrement it in d_free instead
of doing various batching operations to reduce lock hold times in the callers.

Based on an earlier patch from Nick Piggin <npiggin@suse.de>.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/sysctl.c

diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 99a510c..8b77ff5 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1377,7 +1377,7 @@
 		.data		= &dentry_stat,
 		.maxlen		= 6*sizeof(int),
 		.mode		= 0444,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= proc_nr_dentry,
 	},
 	{
 		.procname	= "overflowuid",