LU-9491 llite: Handle multi-vec append write correctly

LU-9491 llite: Handle multi-vec append write correctly

http://review.whamcloud.com/20256 cleaned up LLITE code by using
iov_iter for old and new kernels. However, it introduced a bug to
append write with multiple iovec buffers where each buffer was written
separately therefore each of them saw the same file size.

Append write with multiple buffers has to be performed atomically.

Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: I89d59e924c7029fedc096d724a946763c7f7006d
Reviewed-on: https://review.whamcloud.com/27086
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>