xfs: improve the xfs_dabuf_map calling conventions

Use a flags argument with the XFS_DABUF_MAP_HOLE_OK flag to signal that
a hole is okay and not corruption, and return 0 with *nmap set to 0 to
signal that case in the return value instead of a nameless -1 return
code.

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