LU-2743 mgs: Add a missing rc reset

LU-2743 mgs: Add a missing rc reset

Current mgs_write_log_target() does not reset rc when EALREADY is
found.  This may cause the positive errno to leak to upper layers and
eventually to wire.  This patch adds the missing rc reset.

Change-Id: I0ba8a0decdb9583224a5de02f630742ae94fff39
Signed-off-by: Li Wei <wei.g.li@intel.com>
Reviewed-on: http://review.whamcloud.com/5730
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Tested-by: Hudson
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>