dm space maps: don't reset space map allocation cursor when committing

stable inclusion
from linux-4.19.198
commit e20d40538987b78cdaadcfa2a88344bf97608eb1

--------------------------------

[ Upstream commit 5faafc77 ]

Current commit code resets the place where the search for free blocks
will begin back to the start of the metadata device.  There are a couple
of repercussions to this:

- The first allocation after the commit is likely to take longer than
  normal as it searches for a free block in an area that is likely to
  have very few free blocks (if any).

- Any free blocks it finds will have been recently freed.  Reusing them
  means we have fewer old copies of the metadata to aid recovery from
  hardware error.

Fix these issues by leaving the cursor alone, only resetting when the
search hits the end of the metadata device.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>