LU-5287 export: hold exp_lock when modify exp_flags

There are few places that changing the exp_flags without holding
exp_lock, that could overwrite other concurrent exp_flags updating.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I9c0421ffeacd6bf6feaa31dca9f9fc0e7963bc8b
Reviewed-on: http://review.whamcloud.com/11871
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>