s390/mm: use _SEGMENT_ENTRY_EMPTY in the code

_SEGMENT_ENTRY_INVALID denotes the invalid bit in a segment table
entry whereas _SEGMENT_ENTRY_EMPTY means that the value of the whole
entry is only the invalid bit, as the entry is completely empty.

Therefore we use _SEGMENT_ENTRY_INVALID only to check and set the
invalid bit with bitwise operations. _SEGMENT_ENTRY_EMPTY is only used
to check for (un)equality.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>