librbd: don't temporarily shut down crypto when flattening

librbd: don't temporarily shut down crypto when flattening

(Temporarily) shutting down crypto can lead to data corruption in the
face of concurrent I/O, especially when flatten operation is proxied to
the remote lock owner.  This was added to be able to read, optionally
modify and write crypto header without it being subjected to remapping
and encryption itself.  read_header() and write_header() now achieve
that by specifying CRYPTO_HEADER area explicitly.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>