xfs: clean up xfs_iomap_valid

Rename all iomap_valid identifiers to imap_valid to fit the new
world order, and clean up xfs_iomap_valid to convert the passed in
offset to blocks instead of the imap values to bytes.  Use the
simpler inode->i_blkbits instead of the XFS macros for this.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>