xfs: consolidate preallocation in xfs_file_fallocate

Remove xfs_zero_file_space and reorganize xfs_file_fallocate so that a
single call to xfs_alloc_file_space covers all modes that preallocate
blocks.

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>