LU-15757 llite: check s_root ll_md_blocking_ast()

LU-15757 llite: check s_root ll_md_blocking_ast()

ll_md_blocking_ast() can be called in the context of import
invalidation which in turn caused by umount. this way
ll_md_blocking_ast() and umount can race and ll_md_blocking_ast()
can found sb->s_root NULL which should be checked before
calling into is_root_inode().

Lustre-change: https://review.whamcloud.com/47086/
Lustre-commit: 0095c0d05ca80a2494710e3b4afb1d1e4b5cdcfe

Change-Id: I61c1d29a7de3084ad1dfd0c216cee628418b7038
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48406
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>