mkfs: Sanitise the superblock feature macros

mkfs: Sanitise the superblock feature macros

They are horrible macros that simply obfuscate the code, so
let's factor the code and make them nice functions.

To do this, add a sb_feat_args structure that carries around the
variables rather than a strange assortment of variables. This means
all the default can be clearly defined in a structure
initialisation, and dependent feature bits are easy to check.

[ Eric Sandeen <sandeen@redhat.com>: Properly initialize
	sb_feat.dirftype, remove unnecessary continuation chars. ]

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>