LU-18252 llite: do not unplug the queue on each page

LU-18252 llite: do not unplug the queue on each page

for sync IO ll_write_end() calls into vvp_io_write_commit() at
every page. this get us to:

/* for sync write, kernel will wait for this page to be flushed
 * osc_io_end() is called, so release it earlier.
 * for mkwrite(), it's known there is no further pages. */
if (cl_io_is_sync_write(io) && oio->oi_active != NULL) {
	osc_extent_release(env, oio->oi_active);
	oio->oi_active = NULL;
}

ptlrpcd can be quick enough to intercept current (just reset) extent
and start to process that while the original is still continuing.

in the end this can result in suboptimal RPCs.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: Ie53d0cc7bb52a7ec12b39cfe3da86a623bf03f12
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56449
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>