LU-13309 osd-ldiskfs: remove per-page object_get/put in brw

LU-13309 osd-ldiskfs: remove per-page object_get/put in brw

According to profiling data, object_get/put calls consume a lot
of CPU ticks:

99.99%  [kernel.vmlinux]
           |
           |--28.82%--_atomic_dec_and_lock
           |          |
           |           --28.82%--lu_object_put
           |                     |
           |                      --28.73%--osd_bufs_put
           |                                ofd_commitrw
           |                                tgt_brw_read
           |                                tgt_request_handle
           |                                ptlrpc_server_handle_request
           |                                ptlrpc_main
           |                                kthread
           |                                ret_from_fork
           |
           |--26.51%--osd_bufs_get
           |          |
           |           --26.51%--ofd_preprw
           |                     tgt_brw_read
           |                     tgt_request_handle
           |                     ptlrpc_server_handle_request
           |                     ptlrpc_main
           |                     kthread
           |                     ret_from_fork
           |
           |--18.09%--lu_object_put
           |          |
           |           --18.01%--osd_bufs_put
           |                     ofd_commitrw
           |                     tgt_brw_read
           |                     tgt_request_handle
           |                     ptlrpc_server_handle_request
           |                     ptlrpc_main
           |                     kthread
           |                     ret_from_fork

ofd_preprw_read() and ofd_preprw_write() pin the corresponding
ofd object, later ofd_commitrw_read() and ofd_commitrw_write()
unpin it. When the ofd object is pinned, its underlying
osd object cannot go away, so object_get/object_put
in osd_bufs_get()/osd_bufs_put() are basically no-op.

Change-Id: Ic48f793de5ef3e62505f44879c91050922160000
Signed-off-by: Andrew Perepechko <c17827@cray.com>
Cray-bug-id: LUS-4388
Reviewed-on: https://review.whamcloud.com/37758
Reviewed-by: Alexander Boyko <c17825@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>