s390/zcore: Add hsa file

Under LPAR the zfcpdump HSA is a shared resource. Up to now the HSA memory
is released when the zcore file is closed. Dump programs that know that
they do not need the HSA memory any more (e.g. because they already dumped it)
could release it earlier. This would allow other LPARs to use it again.

To achieve this a new debugfs file "hsa" is added that can be used to read
the HSA size and to release the HSA by writing "0" into the file.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>