LU-17848 osd: fix deref in ldiskfs osd_health_check()

LU-17848 osd: fix deref in ldiskfs osd_health_check()

The implementations of osd_health_check() in ldiskfs
incorrectly check for a NULL mount after already
dereferencing it. Add a check for a NULL mount in
osd_sb() and check for a NULL sb in osd_health_check().

CoverityID: 397885 ("Dereference before null check")

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Id1ce015eb420fe067be375bf0019f305e3e2718c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58989
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lijing Chen <lijinc@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>