LU-5379 ptlprc: return 0 if buf in struct seq_file is overflow

LU-5379 ptlprc: return 0 if buf in struct seq_file is overflow

Former version would get error if the length of content in
nrs_tbf_rul is larger than one page. This patch fix this
problem, now nrs policy tbf could make rules as many as
possible.

Ptlrpc_nrs_policy_control return -ENOSPC means the buf in
parameter struct seq_file is overflow, we return 0 in this
situation to make upper layer function seq_read to alloc a more
larger memory area and do this process again.

Signed-off-by: Wu Libin <lwu@ddn.com>
Change-Id: I436a6b5b58e33e46fb896b6f4d56f64a8ff5a732
Reviewed-on: http://review.whamcloud.com/11154
Tested-by: Jenkins
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Li Xi <pkuelelixi@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>