s390/dasd: put block allocation in separate function

[ Upstream commit 2b43bf06 ]

Put block allocation into a separate function to put some copy pair logic
in it in a later patch.

Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Jan Hoeppner <hoeppner@linux.ibm.com>
Link: https://lore.kernel.org/r/20220920192616.808070-2-sth@linux.ibm.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>
Stable-dep-of: c3116e62 ("s390/dasd: fix double module refcount decrement")
Signed-off-by: Sasha Levin <sashal@kernel.org>