nfs: define nfs_inc_fscache_stats and using it as possible

Define and use nfs_inc_fscache_stats when plus one, which can save to
pass one parameter.

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>