xfs: simplify xfs_iomap_eof_align_last_fsb

By open coding xfs_bmap_last_extent instead of calling it through a
double indirection we don't need to handle an error return that
can't happen given that we are guaranteed to have the extent list in
memory already.  Also simplify the calling conventions a little and
move the extent list assert from the only caller into the function.

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>