mkfs: introduce default configuration structure

mkfs: introduce default configuration structure

mkfs has lots of options that require default values. Some of these
are centralised, but others aren't. Introduce a new structure
designed to hold default values for all the parameters that need
defaults in one place.

This structure also provides a mechanism for providing mkfs defaults
from a config file. This is not implemented in this series, but a
comment is left where it is expected this functionality will hook
in.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>