xfs: make superblock version checks reflect reality

We only support filesystems that have v2 directory support, and than
means all the checking and handling of superblock versions prior to
this support being added is completely unnecessary overhead.

Strip out all the version 1-3 support, sanitise the good version
checking to reflect the supported versions, update all the feature
supported functions and clean up all the support bit definitions to
reflect the fact that we no longer care about Irix bootloader flag
regions for v4 feature bits. Also, convert the return values to
boolean types and remove typedefs from function declarations to
clean up calling conventions, too.

Because the feature bit checking is all inline code, this relatively
small cleanup has a noticable impact on code size:

   text    data     bss     dec     hex filename
 785195  100867     616  886678   d8796 fs/xfs/xfs.o.orig
 783595  100867     616  885078   d8156 fs/xfs/xfs.o.patched

i.e. it reduces it by 1600 bytes.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>