dm: return correct error code in dm_resume()'s retry loop

dm: return correct error code in dm_resume()'s retry loop

commit 8dc23658b7aaa8b6b0609c81c8ad75e98b612801 upstream.

dm_resume() will return success (0) rather than -EINVAL if
!dm_suspended_md() upon retry within dm_resume().

Reset the error code at the start of dm_resume()'s retry loop.
Also, remove a useless assignment at the end of dm_resume().

Fixes: ffcc393641 ("dm: enhance internal suspend and resume interface")
Signed-off-by: Minfei Huang <mnghuan@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>