xfs: simplify the xfs_iomap_write_direct calling

Move the EOF alignment and checking for the next allocated extent into
the callers to avoid the need to pass the byte based offset and count
as well as looking at the incoming imap.  The added benefit is that
the caller can unlock the incoming ilock and the function doesn't have
funny unbalanced locking contexts.

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>