LU-11775 osc: reduce atomic ops in osc_enter_cache_try

LU-11775 osc: reduce atomic ops in osc_enter_cache_try

We can reduce the number of atomic ops performed on
obd_dirty_pages for the common case.

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I5526e449d483241d825af18b612ae1d1dff3241e
Reviewed-on: https://review.whamcloud.com/33859
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>