LU-11282 osd-zfs: drop cache immediately

LU-11282 osd-zfs: drop cache immediately

if this is requested via:
  lctl set_param osd-zfs.*.readcache_max_filesize=<bytes>

dropping cache at read is almost free, but may take few
cycles at write as we have to find corresponding dbufs.

Change-Id: I107fc1bf5a8d7655da4054048ff07d3dffa9d4d8
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/33072
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>