[GFS2] Use a bio to read the superblock

This means that we don't need to create a special inode just to contain
a struct address_space in order to read a single disk block. Instead
we read the disk block directly. Its slightly faster, and uses slightly
less memory, but the real reason for doing this is that it removes a
special case from the glock code.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>