MRP-1267 don't share FS struct for a threads with disk IO.

ldlm_poold, ldlm_elt, mgs_reenqueu threads may do own IO,
but share a same FS structs that may produce a data corruption
with writing to the wrong parttion.

Change-Id: I8278e52275d54741dad4715dac6607bd4e0d0b34
Signed-off-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-on: http://morpheus.xyus.xyratex.com:8443/gerrit/622
Tested-by: Jenkins
Reviewed-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-by: Cheng Shao <cheng_shao@xyratex.com>
Tested-by: Elena Gryaznova <Elena_Gryaznova@xyratex.com>
Reviewed-by: Denis Kondratenko <Denis_Kondratenko@xyratex.com>