ocfs2: Allow direct I/O read past end of file

ocfs2_direct_IO_get_blocks() was incorrectly returning -EIO for a direct I/O
read whose start block was past the end of the file allocation tree. Fix
things so that we return a hole instead. do_direct_IO() will then notice
that the range start is past eof and return a short read.

While there, remove the unused vbo_max variable.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>