LU-13419 osc: Move shrink update to per-write

LU-13419 osc: Move shrink update to per-write

Updating the grant shrink interval is currently done for
each page submitted, rather than once per write.  Since
the grant shrink interval is in seconds, this is
unnecessary.

This came up because this function showed up in the perf
traces for https://review.whamcloud.com/#/c/38151/, and
it is called with the cl_loi_list_lock held.

Note that this change makes this access to the grant shrink
interval a 'dirty' access, without locking, but the grant
shrink interval is:
A) Already accessed like this in various places, and
B) can safely be out of date or suffer a lost update
without affecting correctness or performance.

IOR performance testing with this test:
mpirun -np 36 $IOR -o $LUSTRE -w -t 1M -b 2G -i 1 -F

No patches:
5942 MiB/s
With 38151:
14950 MiB/s
With 38151+this:
15320 MiB/s

Signed-off-by: Patrick Farrell <farr0186@gmail.com>
Change-Id: I8110b3c2570c183d58be2bccdbf76813ea3e373a
Reviewed-on: https://review.whamcloud.com/38214
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>