Set s_flags to 2 to prevent a write on first mount.

The s_flags field is used to indicate whether characters are signed
or unsigned, which differs between ARM and x86 targets. Setting
this value to 0 causes the correct value for the target to be
written back to the superblock on first mount, even if the fs is
marked read only.

Since this value is only used to ensure interoperability with 2.4
series kernels (which we don't need) in dirhash (which we don't
use), we'll just set it to 2 to prevent the write.

Change-Id: I92698235909791fb5f423cfd61ff5299307a8f97