s390/compat,uid16: use current_cred()

86a264ab "CRED: Wrap current->cred and a few other accessors" converted
all uses of current->cred into current_cred() but left s390 alone.

So let's convert s390 finally as well, only five years later.

This way we also get rid of a sparse warning which complains about a
possible invalid rcu dereference which however is a false positive.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>