s390/debug: fix debug area life cycle

[ Upstream commit 9372a82892c2caa6bccab9a4081166fa769699f8 ]

Currently allocation and registration of s390dbf debug areas are tied
together. As a result, a debug area cannot be unregistered and
re-registered while any process has an associated debugfs file open.

Fix this by splitting alloc/release from register/unregister.

Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>