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: NSteven Whitehouse <swhiteho@redhat.com>