mkfs: structify input parameter passing

mkfs: structify input parameter passing

Passing large number of parameters around to number conversion
functions is painful. Add a structure to encapsulate the constant
parameters that are passed, and convert getnum_checked to use it.

This is the first real step towards a table driven option parser.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Jan Tulak <jtulak@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>