LU-13942 obd: check if sbi->ll_md_exp is initialized

LU-13942 obd: check if sbi->ll_md_exp is initialized

Null reference at the start of obd_statfs() function is possible
because of ll_fill_super vs lctl race.

ll_md_exp is initialized in ll_fill_super()->
client_common_fill_super(), but if mount process stucks
in lustre_process_log() it doesn't reach client_common_fill_super().

Change-Id: Ife72a62ba42573e2a9c6d244e36cde738b70c15a
hpe-bug-id: LUS-9150
Signed-off-by: Artem Blagodarenko <artem.blagodarenko@hpe.com>
Reviewed-on: https://es-gerrit.dev.cray.com/157732
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>
Tested-by: Alexander Lezhoev <c17454@cray.com>
Reviewed-by: Andriy Skulysh <c17819@cray.com>
Reviewed-on: https://review.whamcloud.com/39812
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>