[GFS2] Fix bmap to map extents properly

This fix means that bmap will map extents of the length requested
by the VFS rather than guessing at it, or just mapping one block
at a time. The other callers of gfs2_block_map are audited to ensure
they send the correct max extent lengths (i.e. set bh->b_size correctly).

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