ocfs2: Wrap virtual block reads in ocfs2_read_virt_blocks()

The ocfs2_read_dir_block() function really maps an inode's virtual
blocks to physical ones before calling ocfs2_read_blocks().  Let's
extract that to common code, because other places might want to do that.

Other than the block number being virtual, ocfs2_read_virt_blocks()
takes the same arguments as ocfs2_read_blocks().  It converts those
virtual block numbers to physical before calling ocfs2_read_blocks()
directly.  If the blocks asked for are discontiguous, this can mean
multiple calls to ocfs2_read_blocks(), but this is mostly hidden from
the caller.

Like ocfs2_read_blocks(), the caller can pass in an existing
buffer_head.  This is usually done to pick up some readahead I/O.
ocfs2_read_virt_blocks() checks the buffer_head's block number
against the extent map - it must match.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>