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().

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I61c1d29a7de3084ad1dfd0c216cee628418b7038
Reviewed-on: https://review.whamcloud.com/47086
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>