LU-7382 llite: Fix iovec references accounting in ll_file_aio_read/write

LU-7382 llite: Fix iovec references accounting in ll_file_aio_read/write

lti_local_iov is used to store iovec in case of 1 segment.

It is needed to hold reference on lu_env during
call of ll_file_write_iter() or ll_file_read_iter().

Otherwise an assertion fails:
vvp_io_update_iov()) ASSERTION( vio->vui_tot_nrsegs >= vio->vui_iter->nr_segs ) failed

Change-Id: Iaff4c81c6ced9ac0e1557dd0eb1fab5205b48e28
Signed-off-by: Andriy Skulysh <andriy.skulysh@seagate.com>
Reviewed-on: http://review.whamcloud.com/17632
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>