[XFS] Fix a bug in xfs_iomap for extent handling of write cases

This may be the cause of several open PV's of incorrect
delay flags being set and then tripping asserts.
Do not return a delay alloc extent when the caller is asking to do a write.

SGI Modid: xfs-linux:xfs-kern:189616a

Signed-off-by: Russell Cattelan <cattelan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@sgi.com>