LU-5700 llite: handle concurrent use of cob_transient_pages

LU-5700 llite: handle concurrent use of cob_transient_pages

With the lockless __generic_file_aio_write introduced in LU-1669,
ll_direct_IO_26 is no longer protected by the inode i_isem.

This renders obsoltete checks that all transient pages have been
handled before and after entry, and requires atomic access to their
counter.

Signed-off-by: Stephen Champion <schamp@sgi.com>
Change-Id: Ie1545b2123a5ca7d9a8cac130ff387fc06955629
Reviewed-on: http://review.whamcloud.com/12179
Tested-by: Jenkins
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>