gfs2: Stop using GFS2_BASIC_BLOCK and GFS2_BASIC_BLOCK_SHIFT

Header gfs2_ondisk.h defines GFS2_BASIC_BLOCK and GFS2_BASIC_BLOCK_SHIFT
in a misguided attempt to abstract away the fact that sectors on block
devices are 512 or (1 << 9) bytes in size.  Stop using those definitions.

I would be inclinded to remove those definitions altogether, but the
gfs2 user-space tools are using them.

In addition, instead of GFS2_SB(inode)->sd_sb.sb_bsize_shift, simply use
inode->i_blkbits.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Andrew Price <anprice@redhat.com>