um: Remove use of asprinf in umid.c

asprintf is not compatible with the existing uml memory allocation
mechanism. Its use on the "user" side of UML results in a corrupt slab
state.

Fixes: 0d4e5ac7 ("um: remove uses of variable length arrays")
Cc: stable@vger.kernel.org
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>