fs: Protect reconfiguration of sb read-write from racing writes

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

fs: Protect reconfiguration of sb read-write from racing writes

The reconfigure / remount code takes a lot of effort to protect

filesystem's reconfiguration code from racing writes on remounting

read-only. However during remounting read-only filesystem to read-write

mode userspace writes can start immediately once we clear SB_RDONLY

flag. This is inconvenient for example for ext4 because we need to do

some writes to the filesystem (such as preparation of quota files)

before we can take userspace writes so we are clearing SB_RDONLY flag

before we are fully ready to accept userpace writes and syzbot has found

a way to exploit this [1]. Also as far as I'm reading the code

the filesystem remount code was protected from racing writes in the

legacy mount path by the mount's MNT_READONLY flag so this is relatively

new problem. It is actually fairly easy to protect remount read-write

from racing writes using sb->s_readonly_remount flag so let's just do

that instead of having to workaround these races in the filesystem code.

[1] https://lore.kernel.org/all/[email protected]/T/