block/dirty-bitmap: Clean up local variable shadowing

block/dirty-bitmap: Clean up local variable shadowing

Local variables shadowing other local variables or parameters make the
code needlessly hard to understand.  Tracked down with -Wshadow=local.
Clean up: rename both the pair of parameters and the pair of local
variables.  While there, move the local variables to function scope.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20230921121312.1301864-5-armbru@redhat.com>