Adding a percpu_counter nr_dentry avoids cache line ping pongs
between cpus to maintain this metric, and dcache_lock is
no more needed to protect dentry_stat.nr_dentry

We centralize nr_dentry updates at the right place :
- increments in d_alloc()
- decrements in d_free()

d_alloc() can avoid taking dcache_lock if parent is NULL

(socket8 bench result : 27.5s to 25s)

Signed-off-by: Eric Dumazet <dada1-fPLkHRcR87vqlBn2x/YWAg@public.gmane.org>
---
 fs/dcache.c        |   49 +++++++++++++++++++++++++------------------
 include/linux/fs.h |    2 +
 kernel/sysctl.c    |    2 -
 3 files changed, 32 insertions(+), 21 deletions(-)