LU-1128 ldlm: return -1 for server pool shrinker

LU-1128 ldlm: return -1 for server pool shrinker

For ldlm server pool shrinker, we just use it to decrease SLV,
but never reclaim any memory directly, so it should always return
-1 to inform the kernel to break the shrink loop.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I17f51ac84eb0b8c70b2cee9ac7eeca34647c1990
Reviewed-on: http://review.whamcloud.com/2184
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>