LU-4840 lfs: Use file lease to implement migration

LU-4840 lfs: Use file lease to implement migration

Implement non-blocking migration based on exclusive open instead of
group lock. Implemented exclusive close operation to atomically put
a lease, swap two layouts and close a file. This allows race-free
migrations.

Make the caller responsible for retrying on failure (EBUSY, EAGAIN)
in non-blocking mode.

In blocking mode, allow applications to trigger layout swaps using a
grouplock they already own, to prevent race conditions between the
actual data copy and the layout swap. Updated lfs accordingly. File
leases are also taken in blocking mode, so that lfs migrate can issue
a warning if an application attempts to open a file that is being
migrated and gets blocked.

Timestamps (atime/mtime) are set from userland, after the layout swap
is performed, to prevent conflicts with the grouplock.

lli_trunc_sem is taken/released in the vvp_io layer, under the DLM
lock. This re-ordering fixes the original issue between truncate and
migrate.

Signed-off-by: Henri Doreau <henri.doreau@cea.fr>
Signed-off-by: Jinshan Xiong <jinshan.xiong@intel.com>
Change-Id: Ie420e1998cae03928ae24834070c16642e8cd3b9
Reviewed-on: http://review.whamcloud.com/10013
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: frank zago <fzago@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>