LU-12510 osd: osd-zfs to release zrlock quickly

LU-12510 osd: osd-zfs to release zrlock quickly

otherwise few threads trying to access same dnode can get stuck.
this patch is a quick workaround for the issue, it's supposed
to be replaced with a better patch using regular DMU API.

Change-Id: I24d9ed7f8e68080c6a46409476a80799dbb45230
Signed-off-by: Alexey Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35524
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>