Fix cryptfs to work with a raw block device for key storage

Fix cryptfs to work with a raw block device for key storage

If a raw block is specified for key storage, do not try to force the size
of the file to 16 Kbytes when writing the keys, and do not complain if
the size is not 16 Kbytes when reading the keys.  Only do them if the
keyfile is a regular file.

Change-Id: I4de1cb7c3614479d93289d4f2767ca6ce1bbbc73