iomap: iomap_bmap should check iomap_apply return value

Coverity caught this fairly minor bug, but we should check the return
value of iomap_apply regardless.

Coverity-id: 1437065
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>