mon/osd: Deny EC optimizations for non-aligned chunk-sizes and allow switch off.

mon/osd: Deny EC optimizations for non-aligned chunk-sizes and allow switch off.

There are some bugs in the way Fast EC handles non 4k-aligned chunk sizes.

We are unlikely to ever support these and will require any user with these to
use the Umbrella pool migration to switch them off.

If a user of early versions of Tentacle managed to switch this feature on,
we allow the optimizations to be switched off, which should allow the pool
to come back online.

It is generally a bad idea to turn of optimizations, especially if there have
been many writes. However, this should all be detected by scrubbing and the user
is more likely to be able to recover data with the old EC code.

Turning off EC, or turning it on with non-aligned EC sizes requires the
yes-i-really-mean-it flag.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>