chunkfs: chunkfs_create_continuation() use __getname() and drop out more return

We mustn't alloc on stack PATH_MAX bytes for path, instead __getname()
must be used, dot it.
And make code more cleaner by dropping one return statement for success,
use err variable to check whether we need to put cont or not.
chunkfs: chunkfs_get_next_cont(): __getname() instead of allocation on stack