LU-13309 ofd: optimize the brw codepath

LU-13309 ofd: optimize the brw codepath

1) currently, ofd_commitrw() calls lvbo_update()
from the read path with req == NULL.

The only effect of this call is lvb update from
disk. However, the read request does not update
inode info such as atime/mtime/ctime/i_size, so
there's no need for lvb update.

The comment above the removed code claims that
ofd_preprw_read() updated ondisk atime. In fact,
there's no code in the read path which updates
atime. Atime update was implemented by
obdo_to_inode() before we moved to osd.

We don't seem to suffer from this loss, so
attr_get and lvb update, which currently do nothing,
can be simply dropped.

2) ofd_commitrw_*() calls ofd_object_find()
whereas the corresponding preprw call has not
only received the pointer to the object, but
also keeps a reference on it when exits so
ofd_commitrw_*() has to put it twice.

We don't really need to look up the needed
object twice, just record it in the ofd info
and then take it from there.

Change-Id: If9692906e6c8c4e1254d166620d1ea7e68a933b6
Signed-off-by: Andrew Perepechko <c17827@cray.com>
Cray-bug-id: LUS-8127
Reviewed-on: https://review.whamcloud.com/37795
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>