xfs: skip CoW writes past EOF when writeback races with truncate

Every so often we blow the ASSERT(type != XFS_IO_COW) in xfs_map_blocks
when running fsstress, as we do in generic/269.  The cause of this is
writeback racing with truncate -- writeback doesn't take the iolock, so
truncate can sneak in to decrease i_size and truncate page cache while
writeback is gathering buffer heads to schedule writeout.

If we hit this race on a block that has a CoW mapping, we'll get a valid
imap from the CoW fork but the reduced i_size trims the mapping to zero
length (which makes it invalid), so we call xfs_map_blocks to try again.
This doesn't do much anyway, since any mapping we get out of that will
also be invalid, so we might as well skip the assert and just stop.

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