ext4: don't set SB_RDONLY after filesystem errors

In the Linux kernel, the following vulnerability has been resolved:

ext4: don't set SB_RDONLY after filesystem errors

When the filesystem is mounted with errors=remount-ro, we were setting

SB_RDONLY flag to stop all filesystem modifications. We knew this misses

proper locking (sb->s_umount) and does not go through proper filesystem

remount procedure but it has been the way this worked since early ext2

days and it was good enough for catastrophic situation damage

mitigation. Recently, syzbot has found a way (see link) to trigger

warnings in filesystem freezing because the code got confused by

SB_RDONLY changing under its hands. Since these days we set

EXT4_FLAGS_SHUTDOWN on the superblock which is enough to stop all

filesystem modifications, modifying SB_RDONLY shouldn't be needed. So

stop doing that.