s390/extmem: return correct segment type in __segment_load()

stable inclusion
from stable-v5.10.173
commit 6cf48403c46ae4f4ac74a439ca2ff2ddb5eab8c5
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8BFR3

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6cf48403c46ae4f4ac74a439ca2ff2ddb5eab8c5



--------------------------------

commit 8c42dd78 upstream.

Commit f05f62d0 ("s390/vmem: get rid of memory segment list")
reshuffled the call to vmem_add_mapping() in __segment_load(), which now
overwrites rc after it was set to contain the segment type code.

As result, __segment_load() will now always return 0 on success, which
corresponds to the segment type code SEG_TYPE_SW, i.e. a writeable
segment. This results in a kernel crash when loading a read-only segment
as dcssblk block device, and trying to write to it.

Instead of reshuffling code again, make sure to return the segment type
on success, and also describe this rather delicate and unexpected logic
in the function comment. Also initialize new segtype variable with
invalid value, to prevent possible future confusion.

Fixes: f05f62d0 ("s390/vmem: get rid of memory segment list")
Cc: <stable@vger.kernel.org> # 5.9+
Signed-off-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>