afs: afs_vnode_commit_status() doesn't need to check the RPC error

afs_vnode_commit_status() is only ever called if op->error is 0, so remove
the op->error checks from the function.

Fixes: e49c7b2f ("afs: Build an abstraction around an "operation" concept")
Signed-off-by: David Howells <dhowells@redhat.com>