LU-662 fix conflict between seq_client_flush and seq_client_alloc_fid

LU-662 fix conflict between seq_client_flush and seq_client_alloc_fid

seq_client_flush modifies the lu_client_seq.lcs_space by holding the
lu_client_seq.lcs_sem, but seq_client_alloc_fid uses lcs_update to
exclude other concurrent modification.

Change-Id: Ibc3335841b9a045360283e15aab9b6c36b7a39af
Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1364
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>