mds: do not go through handle_mds_failure for oneself

A standby MDS can attempt the handle_mds_failure paths for itself, if
it sees the transition from up to down. This leads it to insert itself
into the resolve_gather set, which is bad. So check if the failed MDS
is the same as whoami, and abort if so. This fixes #4637.

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Yan, Zheng <zheng.z.yan@intel.com>