staging: erofs: rename superblock flags (MS_xyz -> SB_xyz)

This patch follows commit 1751e8a6 ("Rename superblock
flags (MS_xyz -> SB_xyz)") and after commit ("vfs: Suppress
MS_* flag defs within the kernel unless explicitly enabled"),
there is no MS_RDONLY and MS_NOATIME at all.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>