ext4: enable barriers by default

ext4: enable barriers by default

I can't think of any valid reason for ext4 to not use barriers when
they are available;  I believe this is necessary for filesystem
integrity in the face of a volatile write cache on storage.

An administrator who trusts that the cache is sufficiently battery-
backed (and power supplies are sufficiently redundant, etc...)
can always turn it back off again.

SuSE has carried such a patch for ext3 for quite some time now.

Also document the mount option while we're at it.

Signed-off-by: Eric Sandeen <[email protected]>
Signed-off-by: Mingming Cao <[email protected]>
Signed-off-by: "Theodore Ts'o" <[email protected]>