net: devlink: Fix missing mutex_unlock() call

Commit 2dec18ad forgets to call mutex_unlock() before the function
returns in the error path:

   New smatch warnings:
   net/core/devlink.c:6392 devlink_nl_cmd_region_new() warn: inconsistent \
   returns '&region->snapshot_lock'.

Make sure we call mutex_unlock() in this error path.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 2dec18ad ("net: devlink: remove region snapshots list dependency on devlink->lock")
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20220801115742.1309329-1-ammar.faizi@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>