Adding a per_cpu nr_dentry avoids cache line ping pongs between
cpus to maintain this metric.

We centralize decrements of nr_dentry in d_free(),
and increments in d_alloc().

d_alloc() can avoid taking dcache_lock if parent is NULL

Signed-off-by: Eric Dumazet <dada1-fPLkHRcR87vqlBn2x/YWAg@public.gmane.org>
---
 fs/dcache.c        |   55 ++++++++++++++++++++++++++++---------------
 include/linux/fs.h |    2 +
 kernel/sysctl.c    |    2 -
 3 files changed, 40 insertions(+), 19 deletions(-)