btrfs: simplify error path for btrfs_lookup_csums_list()

btrfs: simplify error path for btrfs_lookup_csums_list()

In the error path we have this while loop that keeps iterating over the
csums of the list and then delete them from the list and free them,
testing for an error (ret < 0) and list emptyness as the conditions of
the while loop.

Simplify this by using list_for_each_entry_safe() so there's no need to
delete elements from the list and need to test the error condition on
each iteration.

Also rename the 'fail' label to 'out' since the label is not exclusive
to a failure path, as we also end up there when the function succeeds,
and it's also a more common label name.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>