LU-9224 fid: race between client_fid_fini and seq_client_flush

LU-9224 fid: race between client_fid_fini and seq_client_flush

When the client mount failed or umount, the client_fid_fini() will
be called. At that time, the async connection failure will trigger
seq_client_flush() which parameter may have been released by the
client_fid_fini() by race.

Introduce client_obd::cl_seq_rwsem to protect client_obd::cl_seq.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I96b195676b0d1356a8ae7438cdf69d2e258abd23
Reviewed-on: https://review.whamcloud.com/26079
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>