iomap: support ioends for reads

iomap: support ioends for reads

Support using the ioend structure to defer I/O completion for
reads in addition to writes.  This requires a check for the operation
to not merge reads and writes, and for buffere I/O a call into the
buffered read I/O completion handler from iomap_finish_ioend.  For
direct I/O the existing call into the direct I/O completion handler
handles reads just fine already.

Signed-off-by: Christoph Hellwig <hch@lst.de>