LU-11775 osc: reduce lock contention in osc_unreserve_grant

In osc_queue_async_io() the cl_loi_list_lock is acquired to reserve
and consume the grant and released, right after we expand the extent
the same lock is used to unreserve the grant.
We can keep the spinlock when we are done with the grant to improve
the throughput.

mpirun  -np 32 /root/ior-openmpi/src/ior -w -t 1m -b 8g -F -e -vv
	-o /scratch0/file -i 1
master:
Max Write: 13799.70 MiB/sec (14470.04 MB/sec)
master with 33858:
Max Write: 14339.57 MiB/sec (15036.13 MB/sec)

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: Ic61af84c7b98b5a189d7adabe33ae687954b2ed4
Reviewed-on: https://review.whamcloud.com/33858
Tested-by: Jenkins
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>