md/raid10: fix memleak for 'conf->bio_split'

In the Linux kernel, the following vulnerability has been resolved:

md/raid10: fix memleak for 'conf->bio_split'

In the error path of raid10_run(), 'conf' need be freed, however,

'conf->bio_split' is missed and memory will be leaked.

Since there are 3 places to free 'conf', factor out a helper to fix the

problem.