vfs: in iomap seek_{hole,data}, return -ENXIO for negative offsets

In the iomap implementations of SEEK_HOLE and SEEK_DATA, make sure we
return -ENXIO for negative offsets.

Inspired-by: Mateusz S <muttdini@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>