[S390] personality: fix personality loss on execve

Use the personality() macro to mask out all bits that are not
relevant for the personality type.
The personality field contains bits for other things as well,
so without masking out the not relevalent bits the comparison
won't do what is expected.

Reported-by: Andreas Krebbel <krebbel@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>