Revert "nfs: replace nfs_add_stats with nfs_inc_stats when add one"

This reverts commit 5a254d08.

Since commit 5a254d08 ("nfs: replace nfs_add_stats with
nfs_inc_stats when add one"), nfs_readpage and nfs_do_writepage use
nfs_inc_stats to increment NFSIOS_READPAGES and NFSIOS_WRITEPAGES
instead of nfs_add_stats.

However nfs_inc_stats does not do the same thing as nfs_add_stats with
value 1 because these functions work on distinct stats:
nfs_inc_stats increments stats from "enum nfs_stat_eventcounters" (in
server->io_stats->events) and nfs_add_stats those from "enum
nfs_stat_bytecounters" (in server->io_stats->bytes).

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Fixes: 5a254d08 ("nfs: replace nfs_add_stats with nfs_inc_stats...")
Cc: stable@vger.kernel.org # 3.19+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>