LU-6447 mdt: mdt_identity_upcall to not block with rwlock held

LU-6447 mdt: mdt_identity_upcall to not block with rwlock held

mdt_identity_upcall is currently calling call_usermodehelper
with an rwlock held, which is a no-no since it allocates memory
and schedules. Just replace the rwlock with a rw_semaphore.

Change-Id: I7b063a4db47313fbae6241da7bcec2c397b8e8c4
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-on: http://review.whamcloud.com/14432
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>