btrfs: rename werr and err to ret in __btrfs_wait_marked_extents()

Rename the function's local return variables err and werr to ret.
Also, align the variable declarations with the other declarations in
the function for better function space alignment.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>