btrfs: remove duplicate name variable declarations

When running 'make W=2' there are a few reports where a variable of the
same name is declared in a nested block. In all the cases we can use the
one declared in the parent block, no problematic cases were found.

Reviewed-by: Boris Burkov <boris@bur.io>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>