libblkid: remove unneeded fields from struct bcache_super_block

libblkid: remove unneeded fields from struct bcache_super_block

This started as fix to following warning.

    libblkid/src/superblocks/bcache.c:33:3: warning: ISO C99 doesn’t
    support unnamed structs/unions [-Wc99-c11-compat]

But when sent to review Karel told out rather than getting rid of warnings
lets get rid of everything unnecessary.

[kzak@redhat.com: - use original struct member names rather than 'padN']

Reference: https://github.com/karelzak/util-linux/pull/802
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
Signed-off-by: Karel Zak <kzak@redhat.com>