staging/lustre/llite: take trunc_sem only at vvp layer

The lli_trunc_sem is taken in 'read' mode in both
ll_page_mkwrite and vvp_io_fault_start. This can lead to a
deadlock with another thread which asks for the semaphore
in write mode between thse two read calls.

Since all users of lli_trunc_sem are in the vvp layer, we
can satisfy the requirement to exclude truncate by taking
the semaphore only in vvp_io_fault_start.

Signed-off-by: Patrick Farrell <paf@cray.com>
Reviewed-on: http://review.whamcloud.com/19315
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7981


Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Andriy Skulysh <andriy.skulysh@seagate.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>